'Truthful' in the Bible

The person who has accepted his testimony has acknowledged that God is truthful.

Jesus told them, "What have I been telling you all along? I have much to say about you and to condemn you for. But the one who sent me is truthful, and what I've heard from him I declare to the world."

truthful speech, and divine power; through the weapons of righteousness in the right and left hands;

If we say that we do not have any sin, we are deceiving ourselves and we're not being truthful to ourselves.