In the book of the prophet Isaiah, we find the expression "God of truth" (literally "God of the Amen"), that is, the God who is faithful to his promises: "He who blesses himself in the land shall bless himself by the God of truth [amen]." Our Lord often used the word "Amen," sometimes repeated, to emphasize the trustworthiness of his teaching, his authority founded on God's truth.
Source: Catechism of the Catholic Church (Vatican.va)