How is it that we profess to hold with absolute certitude the revealed truths of our religion? Reasonable enough to say that if your Church is infallible, the doctrines which she preaches are evidently true, and capable of producing absolute certitude in the mind. But the infallibility of your Church is not a self-evident axiom; it is a proposition which you have proved, and proved it by an appeal to ordinary human reason. Is it not clear, then, that in the last resort every statement which your Church makes rests upon the validity of the arguments by which, in the first instance, you proved your Church infallible? Now, these arguments, based as they are upon human reason, do not convey absolute certitude to the mind; they may be, in your view, overwhelmingly probable; nay, they may be certain with all human certainty; but human certainty is not absolute certainty. … You cannot prove the existence of God, the authority of Christ, or his commission to his Church, beyond all possibility of doubt; how then can you suppose that you have proved beyond all possibility of doubt the statements which you receive on the Church's authority?

The motives of credibility, satisfying his intellect, bring the inquirer up to the point of making the act of faith. That act recognizes God's authority in the Church's teaching; and the absolute nature of his authority does make all the difference to the kind of certitude with which, thenceforward, he holds the truths of Catholic doctrine. But this is inherent in the act of faith, not in the chain of proof by which the Catholic claim is established. Having made the act of faith, he cannot produce more or better arguments to convince his neighbour than he could have produced before. Apologetically, then, revealed truths have no higher certitude than the arguments by which the fact of revelation is established. The revealed proposition that there are Three Persons in the Blessed Trinity is not, apologetically, more certain than the statement that our Lord left the charisma of infallibility to his Church. … The absolute certainty with which we believe the teaching of the Church comes to us from the supernatural grace of faith, which transforms our reasoned conviction into a higher quality - the water, as at Cana, is turned into wine.

- Msgr. Ronald Knox, The Belief of Catholics