To which church does Jesus introduce Himself as "the Amen, the faithful and true Witness"?

Answer: Laodicea

The letter to Laodicea begins: "These are the words of the Amen, the faithful and true Witness."

“To the angel of the church in Laodicea write: These are the words of the Amen, the faithful and true Witness, the Originator of God’s creation.”— Revelation 3:14 (BSB)