The Creed, like the last book of the Bible, ends with the Hebrew word amen. This word frequently concludes prayers in the New Testament. the Church likewise ends her prayers with “Amen.”
In Hebrew, amen comes from the same root as the word “believe.” This root expresses solidity, trustworthiness, faithfulness. and so we can understand why “Amen” may express both God’s faithfulness towards us and our trust in him.
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.
Thus the Creed’s final “Amen” repeats and confirms its first words: “I believe.” To believe is to say “Amen” to God’s words, promises and commandments; to entrust oneself completely to him who is the “Amen” of infinite love and perfect faithfulness. the Christian’s everyday life will then be the “Amen” to the “I believe” of our baptismal profession of faith:
May your Creed be for you as a mirror. Look at yourself in it, to see if you believe everything you say you believe. and rejoice in your faith each day.
Jesus Christ himself is the “Amen.” He is the definitive “Amen” of the Father’s love for us. He takes up and completes our “Amen” to the Father: “For all the promises of God find their Yes in him. That is why we utter the Amen through him, to the glory of God”:
Through him, with him, in him,
in the unity of the Holy Spirit,
all glory and honor is yours,
God, for ever and ever.
From the Catechism of the Catholic Church 1061-1065