Catechism of the Catholic Church

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. (215, 156)