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,
almighty Father,
God, for ever and ever.
AMEN.
Source: Catechism of the Catholic Church (Vatican.va)