What does it mean to say that God is truth?
"God is light and in him is no darkness" (1 Jn 1:5). His Word is truth (Prov 8:7; 2 Sam ), and his Law is truth (Ps 119:142). Jesus himself vouches for God's truth by declaring before Pilate, "For this I was born, and for this I have come into the world, to bear witness to the truth" (Jn ).
The truth of God cannot be "proved" or subjected to empirical tests, since science cannot make him an object of investigation. And yet God subjects himself to a special sort of proof. We know that God is truth on the basis of the absolute credibility of Jesus. He is "the way, and the truth, and the life" (Jn 14:6). Anyone who enters into a relationship with him can find this out by putting it to the test. If God were not "true", then faith and reason could not have a dialogue with each other. An agreement is possible, however, because God is truth, and the truth is divine.
(From the Catholic Youth Cayechism question 32)