"the Establishment, for instance, accepts from the Catholic Church the doctrine of the Incarnation; but at the same time denies that Christ is in the Blessed Sacrament and that Mary is the Mother of God; who in consequence will venture to affirm that such of its members as hold the Incarnation, hold it in virtue of their membership? the Establishment cannot really hold a Catholic doctrine, a portion and a concomitant of which it puts on one side. The Incarnation has not the same meaning to one who holds and to one who denies these two attendant verities. Hence, whatever he may profess about the Incarnation, the mere Protestant has no real hold, no grasp of the doctrine; you cannot be sure of him; any moment he may be found startled and wondering, as at a novelty, at statements implied in it, or uttering sentiments simply inconsistent with its idea. Catholicism is one whole, and Protestantism has no part in it."
John Henry Newman, The idea of a university defined and illustrated, Discourse V ("General knowledge viewed as one philosophy"), ed. I. T. Ker (Oxford: The Clarendon Press, 1976), 429 (Appendix I). Newman then goes on to make the same point about Islam (shades of the 2015-2016 Wheaton College Larycia Hawkins controversy).
"any moment he may be found startled and wondering, as at a novelty, at statements implied in it": as was I the day I (raised Quaker) realized that God is still (and will be forever) incarnate (a truth brought home to me, it should be admitted, by a Presbyterian seminary professor); as were two members of my extended (and at least historically Quaker) family on the day that, many years later, I made that same claim in their presence.
Cf. this nearby passage.