The Enchiridion 5

Moreover, when the mind has been imbued with the first elements of that faith which worketh by love, it endeavors by purity of life to attain unto sight, where the pure and perfect in heart know that unspeakable beauty, the full vision of which is supreme happiness. Here surely is an answer to your question as to what is the starting-point, and what the goal: we begin in faith, and are made perfect by sight. This also is the sum of the whole body of doctrine. But the sure and proper foundation of the catholic faith is Christ. “For other foundation,” says the apostle, “can no man lay than that is laid, which is Jesus Christ.” Nor are we to deny that this is the proper foundation of the catholic faith, because it may be supposed that some heretics hold this in common with us. For if we carefully consider the things that pertain to Christ, we shall find that, among those heretics who call themselves Christians, Christ is present in name only: in deed and in truth He is not among them. But to show this would occupy us too long, for we should require to go over all the heresies which have existed, which do exist, or which could exist, under the Christian name, and to show that this is true in the case of each,—a discussion which would occupy so many volumes as to be all but interminable.