God revealed the Faith to us, and He is trustworthy. This is the foundational principle that unifies the whole deposit of the Faith. We cannot be united to God unless we believe He is an all-good, all-loving, all-knowing, and all-powerful Father. Once we start to pick and choose between aspects of the Faith, and decide to believe only the parts we wish to, we by this very fact either say by our choices that God is not trustworthy, doesn't know what He is talking about, or is misleading us. St. Augustine says "If you believe what you like in the Gospels, and do not believe what you dislike, it is not the Gospels you believe but yourself." St. Thomas Aquinas' beautiful hymn Adoro Te Devote speaks beautifully to this:
"Seeing, touching, tasting are in Thee deceived:
How says trusty hearing? that shall be believed;
What God's Son has told me, take for truth I do;
Truth Himself speaks truly or there's nothing true."