Hi there,
So there are truths one knows by reason and truths one knows by revelation.
Truths such as God exists can be known by reason. So how Christian faith works is sort of like the following
- God exists → Reason
- Christ existed, died and rose from the dead. Therefore he is who he said he was i.e. Son of God → History and Reason
- Tradition → Teachings by the son of man, interpreted and spread forth by the Apostles/Disciples
- Scripture → Written part of Tradition
- Church → Instituted by Jesus (known through Reason and using Scripture and Tradition as merely historical texts describing the early church)
- All truths declared by the church → Since we believe in the church
So when we approach truths like the Trinity, we accept it by faith as revelation as the Church tells us. We have no reason to question the Church. One can see how the church arrived at the conclusion. One can think of ways to understand how the Trinity could be. But one does not have to prove the Trinity to believe in the Trinity.
Since a Catholic has accepted Scripture, Tradition and Church as authority, it would be UNREASONABLE to doubt any of the truths declared by the Church infallibly.
God Bless