There’s philosophical truth, which we can know by reason alone, and divine truth, which we can only know because God reveals it to us. Much of what we know about God is philosophical truth, but everything that we know about God’s will is divine truth.
Catholics believe that God communicates divine truth through the teaching authority of the Catholic Church, and this comprises the doctrine of the Church. Because it is divine truth, it may never be in error and it may never change, although it may grow and develop into a more complete understanding.
FWIW, divine truths are sometimes called “mysteries,” though not with the usual English understanding of the word. We think of a mystery as something we don’t know, but the Church uses the word to describe things that God has told us about. This can be confusing to some English speakers.
