When we are dealing with religious truths taught by Revelation, we have facts which in most cases we cannot prove. We may be able to give a reasonable justification for our belief, to show that we are acting sensibly in believing, and to show that what we believe is reasonable, but we cannot do more. What we accept, we accept as matters of faith, as truths taught us by God Himself. When a man accepts the Bible as the word of God, therefore, or any other particle of religious teaching, that very acceptance is his creed. The first thing that sets the Creed of the Catholic Church apart from all others is that it is definite.
Catholics believe that truth is one, absolute and unchanging.
That is merely to say that Catholics believe that things are what they are. A thing is what it is, and it is nothing else. A thing either is, or it is not. There are no other alternatives. If a thing is true, it cannot also be false.