It could credibly be argued that the first seeds of the Second Vatican Council were sown on 2 February 1843, when the then-Anglican John Henry Newman preached a sermon in Oxford under the title of ‘The Theory of Developments in Religious Doctrine’. 
Taking as his text Luke 2:19, ‘Mary kept all these things, and pondered them in her heart,’ Newman argued that a truly Christian faith is a Marian faith, not merely accepting what has been revealed, but reflecting upon it, using it, developing it, reasoning on it. Describing as ‘wonderful’ the development and growth of the Christian mind, Newman said:
‘And this world of thought is the expansion of a few words, uttered, as if casually, by the fishermen of Galilee. … Reason has not only submitted, it has ministered to Faith; it has illustrated its documents; it has raised illiterate peasants into philosophers and divines; it has elicited a meaning from their words which their immediate hearers little suspected. … Its half sentences, its overflowings of language, admit of development; they have a life in them which shows itself in progress; a truth, which has the token of consistency; a reality, which is fruitful in resources; a depth, which extends into mystery: for they are representations of what is actual, and has a definite location and necessary bearings and a meaning in the great system of things, and a harmony in what it is, and a compatibility in what it involves.’
Drawing on an earlier distinction between what he deemed ‘explicit’ and ‘implicit’ reason, Newman argued that Revelation impresses certain supernatural facts or principles on the minds of those to whom truth is revealed, holding that those upon whose minds these supernatural realities had been impressed could be unaware of the truths which they possessed, such that over time they would draw unconsciously on realities they could not articulate, and ‘centuries might pass without the formal expression of a truth, which had been all along the secret life of millions of faithful souls’.
Two years later, in his Essay on the Development of Christian Doctrine, Newman developed this thesis at much greater length, addressing the apparently undisputable historical reality that Christian teaching had varied so much over the centuries that one might legitimately wonder whether there had been any true ‘continuity of doctrine’ since Apostolic times. 
Newman argued that a true continuity of doctrine could indeed be discerned, with any appearances to the contrary to be expected, given that Christianity is a living thing; butterflies do not obviously resemble the caterpillars from which they grow, after all, but the butterfly is, as it were, ‘written’ in the caterpillar and should be regarded as its authentic and flourishing mature form, just as the chicken is written in the egg, and the mustard bush in the proverbial mustard seed. In a famous passage he wrote that while it is sometimes said that streams are clearest near where they rise, this is not quite true for the history of philosophy or belief, which:
‘… is more equable, and purer, and stronger, when its bed has become deep, and broad, and full. It necessarily rises out of an existing state of things, and for a time savours of the soil. Its vital element needs disengaging from what is foreign and temporary.’
Newman had long believed growth, as he remarks in his spiritual autobiography, ‘the only evidence of life’, and as he regarded the Church as a living thing, so he regarded development of Christian tradition as inevitable. To be faithful, however, a development must retain ‘both the doctrine and the principle with which it started.’As a living thing, however, it was prone to develop in an organic fashion:
‘From time to time it makes essays which fail and are in consequence abandoned. It seems in suspense which way to go; it wavers, and at length strikes out in one definite direction. In time it enters upon strange territory; points of controversy alter their bearing; parties rise and fall around it; dangers and hopes appear in new relations; and old principles reappear under new forms. It changes with them in order to remain the same. In a higher world it is otherwise, but here below to live is to change, and to be perfect is to have changed often.’
This notion of organic development was perhaps Newman’s greatest contribution to Christian thought. Hitherto there had been a number of ways of addressing the question of doctrinal development, none of which Newman found satisfactory: some Protestants believed that Christianity had only developed by absorbing foreign elements, which necessitated a return to a Bible-only religion and seemed to contradict the guarantees Christ had given his Church, while Anglicans tended to favour the principle of St Vincent of Lerins that Christianity is ‘what has been held always, everywhere, and by all’, which Newman felt unworkable and inclined to undercut all Christian groups without exception. Catholic theories on development tended to hold that all doctrines had always been explicitly present even if secretly so, but the Scholastic theory of logical explication based on deductions from earlier formulations did not fit easily with the known facts of history, and Bossuet’s principle of clarification which saw developments as later explanations of earlier formulations did not really explain how so much development had demonstrably taken place.
Just as Newman’s contemporary Darwin was not the first to envisage some form of development of species, so Newman was not the first to envisage some form of development of doctrine; what was new, however, was his belief that doctrine developed organically, with the faithful reason of believers working over centuries under the guidance of the Holy Spirit to nurture and polish the original revelation so that it became a divine philosophy. That Newman should have thought in this ‘evolutionary’ fashion is hardly surprising given how ‘progress’ was the central theme in mid-nineteenth century thought; the Industrial Revolution had dramatically changed technology, culture, and society, such that scholars and intellectuals of all sorts wrestled with how economies, life, personalities, and ideas develop. 
Newman may have regarded the process of development as organic, but he certainly did not believe it aimless or random; if he thought Vincent of Lerins’ approach unworkable, nonetheless it had value in how it defined authentic development in doctrine as ‘a real progress for the faith, and not an alteration: the characteristic of progress being that each element grows and yet remains itself, while the characteristic of alteration is that one thing is transformed into another.’ As Newman understood it, true developments retained both the original doctrine and the original principle.
Having early in his Anglican career regarded dogma as a mere necessary evil, taking the view that ideally Christianity would be – as it surely was in its earliest years – simple and free of such clutter, by the time he came to write his Grammar of Assent he had come to believe that the supposition that there was ‘a contrariety and antagonism between a dogmatic creed and vital religion’ was simply false. He explained that dogma ascertains and makes clear ‘the truths on which the religious imagination has to rest’, as ‘knowledge must ever precede the exercise of the affections’; emotional and imaginative sentiment, then, depend on the intellect, and as such devotion depends upon dogma.
If it might seem surprising that this could ever have proved congenial to him, given his earlier views, it is worth turning to G.K. Chesterton, the final chapter of whose Orthodoxy seems to owe a clear debt to Newman’s Grammar. In this, Chesterton observes that Christianity needs doctrine if it is to flourish and to be free:
‘Catholic doctrine and discipline may be walls; but they are the walls of a playground. […] We might fancy some children playing on the flat grassy top of some tall island in the sea. So long as there was a wall round the cliff’s edge they could fling themselves into every frantic game and make the place the noisiest of nurseries. But the walls were knocked down, leaving the naked peril of the precipice. They did not fall over; but when their friends returned to them they were all huddled in terror in the centre of the island; and their song had ceased.’
Given this, the teaching duty of the Church, for Newman, could hardly have been clearer. To a Catholic, wrote Newman to Richard Holt Hutton in 1871, the Church is, so to speak, ‘a standing Apostolic committee – to answer questions, which the Apostles are not here to answer, concerning what they received and preached.’ Not knowing more than the Apostles, he explained, there are questions the Church cannot answer, but it nonetheless was empowered to state the doctrine of the Apostles, ‘what is to be believed, and what is not such’. 
This imposed a responsibility on the Magisterium so fearful that, Newman believed, occasional excesses of zeal on the part of the Church’s doctrinal watchdogs were as understandable as they were unavoidable:
‘In this curious sceptical world, such sensitiveness is the only human means by which the treasure of faith can be kept inviolate. There is a woe in Scripture against the unfaithful shepherd. We do not blame the watch-dog because he sometimes flies at the wrong person. I conceive the force, the peremptoriness, the sternness, with which the Holy See comes down upon the vagrant or the robber, trespassing upon the enclosure of revealed truth, is the only sufficient antagonist to the power and subtlety of the world, to imperial comprehensiveness, monarchical selfishness, nationalism, the liberalism of philosophy, the encroachments and usurpation of science.’
Occasional bouts of hypervigilance, then, however regrettable, were a price worth paying if the integrity of the Faith was to be protected. Not, of course, that it was for the Holy See alone to guard the deposit of faith. One of the other great themes in Newman’s writing, and one which flourished in the Second Vatican Council, was the role of the laity in preserving the truth that had been revealed; as early as 1835 Newman remarked of the laity to his friend Richard H. Froude that ‘the maintenance of the faith is their clear prerogative’.
Clearly seeing the laity as an essentially conservative body, in 1859’s On Consulting the Faithful in Matters of Doctrine Newman argued forcefully that it was appropriate for Rome to take into account what the laity believed on issues as yet undefined. It was wise to do this, he said, ‘because the body of the faithful is one of the witnesses to the fact of the tradition of revealed doctrine, and because their consensus through Christendom is the voice of the Infallible Church.’
When Newman spoke of the laity being consulted, he stressed, he did not mean that the Magisterium should seek their opinion on how Rome should define things; rather, he said, the Holy See should consult the laity as a man would consult a barometer or a railway timetable, as a simple matter of fact: the question on any given doctrine was not ‘what does the laity believe the Church should teach?’ so much as ‘what does the laity see Church teaching as being?’
Newman was careful to speak, too, of the laity as a whole, referring to the consensus fidelium, the shared mind of the faithful throughout the world. He was all too aware of how portions of the laity could be out of step with the mind of the universal Church, noting, for instance, in his lectures on The Present Position of Catholics in England, that:
‘In all times the laity have been the measure of the Catholic spirit; they saved the Irish Church three centuries ago and they betrayed the Church in England. Our rulers were true, our people were cowards.’
Insofar as the Church’s infallibility subsisted in the laity, then, it did so in a universal, not a sectional sense, and depended to an enormous – perhaps to an absolute – degree on how effectively and thoroughly they had been raised and formed in the truths of the Faith. Newman’s views on the laity as an authentic channel of tradition had been formed by his studies of the Arian heresy and how it was received by the fourth-century Church. Distinguishing between the part of the Church that teaches and the part of the Church that is taught, Newman maintained that the fourth-century Church leadership had hardly covered itself in glory, whereas the sort of well-instructed laity for which he hoped in his own day resisted the Arian innovations and maintained the true doctrine of the Church:
‘For I argue that, unless they had been catechised, as St Hilary says, in the orthodox faith from the time of their baptism, they never could have had that horror, which they show, of the heterodox Arian doctrine. Their voice, then, is the voice of tradition…’
In short, then, Newman regarded doctrine as something that developed but did so organically, developments arising and being embraced gradually after centuries of reflection, these developments being signs of true growth, rather than the kind of changes that would change the essential character of things.
Theologians had the job of thinking and pondering on what the Church believed, but it was not for them to steer Peter's barque; rather, that task was primarily that of the Magisterium, with the Pope's primary role being to ensure the Church's unity in truth. If theologians should step outside the boundaries of the Church's belief, or threaten to lead others outside the established limits of Christian truth, then it was the duty of Rome to step in. Doing otherwise would be to neglect Rome's pastoral duties: good shepherds try to prevent sheep from straying, and strive to bring back lost sheep.
As for the laity, Newman saw theirs as an essentially conservative role, their job being in large part to watch the watchmen, to preserve the historical faith and practice of the Church -- because practice does not merely reflect faith, but can drive faith, with changes in practice leading to changes in faith -- even when clergy and theologians discard and deny what the Church had long believed and done. But the laity could not do this job, Newman was clear, unless they knew and embraced the Church's authentic historical belief; without a well-catechised laity, there could be no true sensus fidelium. Proper instruction in the realities of the Catholic faith, then, is vital if the laity is to be empowered, and health of the Church to be robust.
 Ker, Ian, John Henry Newman, Oxford 1988, 266-269; Ker, Ian, Newman on Being a Christian, London 1990, 29-31; Newman, John Henry, Oxford University Sermons, London 1900, 312-351. Development of doctrine was arguably the central issue at the Second Vatican Council, with Newman’s work in this area being regarded as foundational, according to O’Malley, John W., What Happened at Vatican II, Cambridge MA 2008, 39.
 Newman, Oxford University Sermons, 313. Congar, Yves, The Meaning of Tradition, trans. A.N. Woodrow, San Francisco 2004, 112 likewise presents Mary as the archetype of the reflective Church, citing Hugo Rahner’s description of the Church as ‘the Mary of the history of the world’.
 Newman, Oxford University Sermons, 317-318.
 Newman, Oxford University Sermons, 251-277.
 Newman, Oxford University Sermons, 320-323.
 Newman, John Henry, An Essay on the Development of Christian Doctrine, London 1973, 69-72 notes his belief that the human and institutional continuity that was so clear in Christian history pointed strongly to the likelihood of real doctrinal continuity, but also admits the possibility that over time the ‘blade’ and ‘handle’ of Christianity might have been changed so often that doctrinal change could have happened without matters looking any different.
 Newman, Development, 117; on mustard seeds, see, of course, Matthew 13:31-32, Mark 4:30-32, and Luke 13:18-19.
 Newman, Development, 100.
 Newman, John Henry, Apologia Pro Vita Sua, London 1962, 99.
 Newman, Development, 129.
 Newman, Development, 100.
 Bieber, Newman on Tradition, 131; Newman, Development, 88.
 Ker, John Henry Newman, 302; Newman, Development, 74-88.
 Bieber, Newman on Tradition, 131-132; Ker, Newman on Being a Christian, 31-32; Newman, Development, 88-90.
 Bieber, Günter, Newman on Tradition, translated by Kevin Smyth, London 1967, 130.
 Elsdon-Baker, Fern, The Selfish Genius: How Richard Dawkins rewrote Darwin’s Legacy, London 2009, 72.
 As quoted in Congar, Meaning of Tradition, 119.
 Ker, Newman on Being a Christian, 22; Newman, John Henry, The Arians of the Fourth Century, London 1890, 36-37.
 Newman, John Henry, The Grammar of Assent, ed. I.T. Ker, Oxford 1985, 82-83. Ker, Newman on Being a Christian, 25-26.
 Chesterton, G.K., Orthodoxy in G.K. Chesterton Collected Works Volume I, San Francisco 1986, 348-349 reads as a personal take on Newman’s arguments for belief in Christianity based on natural inference and the illative sense. See also Oddie, William, Chesterton and the Romance of Orthodoxy: The Making of GKC 1874-1908, Oxford 2008, 362-363 on Chesterton as a disciple of Newman in this respect.
 Chesterton, Orthodoxy, 350.
 Dessain, Charles Stephen and Thomas Gornall, eds, The Letters and Diaries of John Henry Newman – Volume XXV: The Vatican Council January 1870 to December 1871, Oxford 1973, 418.
 Newman, John Henry, On Consulting the Faithful in Matters of Doctrine, London 1961, 61.
 Bieber, Newman on Tradition, 39.
 Newman, John Henry, On Consulting the Faithful, 63. On the witnesses to Christian tradition, see Congar, Meaning of Tradition, 129-155, especially 140: ‘Living tradition, faithfully lived by Christians, is not creative, but is, in a sense, a source of Revelation – precisely because it contains and makes explicit things that it has always held and practiced concretely, but for which, in the beginning, there existed no written or verbal formulation.’
 Newman, John Henry, Lectures on the Present Position of Catholics in England, London 1896, 390-391; Newman’s comments on the cowardice of the English laity must be seen not as an absolute criticism, but as a comparative one, in contrast to the faith of their Irish brethren. His observation on the courage of their ecclesiastical leaders seems broadly fair; Eamon Duffy, Fires of Faith: Catholic England under Mary Tudor, New Haven 2009, #, notes that 10 of the 23 bishops Mary inherited from Edward VI returned to unity with the Catholic Church, with all bar one of the bishops she bequeathed Elizabeth rejecting the Elizabethan settlement, and observes that more than two thirds of Edward’s clergy returned to the Catholic Church under Mary, many of them retaining this allegiance after her death, noting that, for instance, it was possible in 1561 to walk through sixteen parishes between the Surrey border and the Sussex coast, in each of which the incumbent had either died of influenza or been deprived of office for refusing to conform to the Elizabethan settlement.
 Newman, Consulting the Faithful, 76; on Newman’s desire for an ‘intelligent, well-instructed laity’, see, famously, Newman, Present Position, 390.