Why did God have to show himself in order for us to be able to know what he is like?
Man can know by reason that God exists, but not what God is really like. Yet because God would very much like to be known, he has revealed himself.
God did not have to reveal himself to us. But he did it out of love. Just as in human love one can know something about the beloved person only if he opens his heart to us, so too we know something about God’s inmost thoughts only because the eternal and mysterious God has opened himself to us out of love. From creation on, through the patriarchs and the prophets down to the definitive revelation in his Son Jesus Christ, God has spoken again and again to mankind. In him he has poured out his heart to us and made his inmost being visible for us
CCC section (50-53)