From Mary Kassian’s Knowing God by Name:
The Hebrew words for faithful and true are both derivatives of the verb Aman, whence the word Amen draws its origin. “The God of Amen” is an Old Testament title for God (Isa. 65:16). In the New Testament, Jesus often introduced His statements with the formula, “Amen (verily/truthfully) I say unto you…” In doing so, He identified himself with God and revealed Himself as the One through whom all of God’s promises are fulfilled.
In the Book of Revelation, Christ is called “the Amen, the faithful and true witness” (Rev 3:14). Paul explained why: “All the promises of God find their Yes in him. That is why it is through him that we utter our Amen to God for his glory” (2 Cor. 1:20).
Have you ever wondered what saying “Amen” at the end of a prayer means? When you say “Amen” you are essentially saying “Faithful!” “True!” “Verified!” “Yes! Yes!! YES!!!” Your “Amen” affirms that you believe that God’s character is trustworthy, that God is your pillar, that you are confident He will hear your prayer, be faithful and true to do what He promised, and fulfill His purposes in your life.
The Lord’s faithfulness is established (Ps. 89:2), incomparable (Ps. 89:8), unfailing (Ps. 89:33), infinite (Ps. 36:5), and everlasting (Ps. 146:6). And even if we mess up and unfaithful to Him, His faithfulness to us will endure. Scripture says, “If we are faithless, he remains faithful–for he cannot deny himself” (2 Tim. 2:13). His character is totally and completely trustworthy. Isn’t that an amazing thought? What an awesome God He is! What a GOOD God He is!