John Gill Commentary Revelation 22:20

"He who testifieth these things saith, Yea: I come quickly. Amen: come, Lord Jesus." — Revelation 22:20 (ASV)

He which testifies these things
Not the angel (Revelation 22:16), nor John, who sometimes used this way of speaking (John 21:24), but Christ, as appears from what follows;

for he says, surely I come quickly ;
who not only asserts the speediness of his coming, as in (Revelation 22:7Revelation 22:12), but expresses the certainty of it, so that it ought not to be doubted of by any, especially by his own people; though it may seem to be deferred, and, upon that account, be derided by ungodly men; and Christ may have some respect, in this strong repetition of it, to the certainty of the punishment that will then be inflicted upon such who add to, or diminish from this book; for he himself will certainly come in person, and execute the vengeance threatened.

John next puts his Amen
to it, signifying his assent to it, declaring his faith in it, and expressing his earnest desire after it, and wish for it: and in words at length adds, even so come, Lord Jesus ;
quickly, speedily as you have said, and in all your glory; set up your kingdom, let that come, introduce your people into it, and destroy your enemies; this he said as one that loved the appearance of Christ, longed for it, hastened to it, and was impatient at the delay of it.