Why is Jesus called "the Amen" in Revelation 3:14?

ὁ ἀμήν ("the Amen") ὁ μάρτυς ὁ πιστὸς καὶ ἀληθινός ("the faithful and true witness") ἡ ἀρχὴ τῆς κτίσεως τοῦ Θεοῦ· ("the beginning of God's creation")