The word amen
speaks to the certainty and the reliability of words just spoken. Jesus had earlier described himself as the Amen, the faithful and true witness
(Revelation 3:14). In the present verse, the same John who received and recorded the Lord’s revelation responds to Jesus’ promise to come soon with a hearty amen.
With that response, John expresses his conviction that Christ is witnessing truthfully to the reality of his imminent return.
20 He who testifies to these things says, “Surely I am coming soon.” Amen. Come, Lord Jesus!