" much more natural will be the transition from one religion to another, without injury to existing certitudes,when the common points, the objects of... certitudes,are truths; and still stronger in that case and more constraining will be the sympathy, with which minds that love truth,even when they have surrounded it with error, will yearn towards the Catholic faith,which contains within itself, and claims as its own, all truth that is elsewhere to be found, and more than all, and nothing but truth. This is the secret of the influence, by which the Church draws to herself converts from such various and conflicting religions. They come, not so much to lose what they have, as to gain what they have not; and in order that, by means of what they have, more may be given to them. St Augustine tells us that there is no false teaching without an intermixture of truth; and it is by the light of those particular truths, contained respectively in the various religions of men, and by our certitudes about them, which are possible wherever these truths are found, that we pick our way slowly perhaps, but surely, into the One Religion which God has given, taking our certitudes with us, not to lose, but to keep them more securely, and to understand and love their objects more perfectly.
.....And thus it is conceivable that a man might travel in his religious profession all the way from heathenism to Catholicity, through Mahometanism, Judaism, Unitarianism, Protestantism and Anglicanism, without any one certitude lost, but with a continual accumulation of truths, which claimed from him and elicited in his intellect fresh and fresh certitudes."
John Henry Newman, The Grammar of Assent