1. I am far more certain (according to the Fathers) that we are in a state of culpable separation, than that developments do not exist under the Gospel, and that the Roman developments are not the true ones. 2. I am far more certain, that our (modern) doctrines are wrong, than that the Roman (modern) doctrines are wrong. 3. Granting that the Roman (special) doctrines are not found drawn out in the early Church, yet I think there is sufficient trace of them in it, to recommend and prove them,on the hypothesis of the Church having a divine guidance, though not sufficient to prove them by itself. So that the question simply turns on the nature of the promise of the Spirit, made to the Church. 4. The proof of the Roman (modern) doctrine is as strong (or stronger) in Antiquity, as that of certain doctrines which both we and Romans hold: e.g. there is more of evidence in Antiquity for the necessity of Unity, than for the Apostolical Succession; for the Supremacy of the See of Rome, than for the Presence in the Eucharist; for the practice of Invocation, than for certain books in the present Canon of Scripture, &c. &c. 5. The analogy of the Old Testament, and also of the New, leads to the acknowledgment of doctrinal developments."
<4.> And thus I was led on to a further consideration. I saw that the principle of development not only accounted for certain facts, but was in itself a remarkable philosophical phenomenon, giving a character to the whole course of Christian thought. It was discernible from the first years of the Catholic teaching up to the present day, and gave to that teaching a unity and individuality. It served as a sort of test, which the Anglican could not exhibit, that modern Rome was in truth ancient Antioch, Alexandria, and Constantinople, just as a mathematical curve has its own law and expression.
<5.> And thus again I was led on to examine more attentively what I doubt not was in my thoughts long before, viz. the concatenation of argument by which the mind ascends from its first to its final religious idea; and I came to the conclusion that there was no medium, in true philosophy, between Atheism and Catholicity, and that a perfectly consistent mind, under those circumstances in which it finds itself here below, must embrace either the one or the other. And I hold this still: I am a Catholic by virtue of my believing in a God; and if I am asked why I believe in a God, I answer that it is because I believe in myself, for I feel it impossible to believe in my own existence (and of that fact I am quite sure) without believing also in the existence of Him, who lives as a Personal, All-seeing, All-judging Being in my conscience.
Apologia Chapter 6