With these words John adds to the conviction captured in the word amen.
That is, not only is he sure that Christ is coming as soon as possible but he also prays back to Jesus his promise to come soon. It’s a petition that the Lord would indeed do as he has repeatedly said he’d do. We realize that John’s petition would—and should—receive echo from the mouths and hearts of all who heard this revelation.
20 He who testifies to these things says, “Surely I am coming soon.” Amen. Come, Lord Jesus!