For I testify unto every man that heareth the words of the prophecy of this book, If any man shall add unto these things, God shall add unto him the plagues that are written in this book: (Revelation 22:18)
The speaker is Jesus (see v. 20). His testimony must be accepted.
God shall add.
In justice, God can do no other than to give to each his due, in harmony with his works.
Add.
Compare Deut. 4:2; 12:32. Jesus is authenticating this book of the Revelation; He warns against deliberate changes in the message of the book. Josephus says concerning the 22 books constituting the Hebrew OT; “For, although such long ages have now passed, no one has ventured either to add, or to remove, or to alter a syllable” (Against Apion i. 8 [42]; Loeb ed., pp. 179, 180).
Of the prophecy.
John is speaking of additions to the book of Revelation, though the same would apply to any book of the Sacred Canon.
Heareth.
Not a reference to the mere falling of the sound of the words of this book upon one’s ear; rather, an allusion to one who hears and studies the import of the messages (see on ch. 1:3).
Every man.
Man’s relation to God and His message is a personal matter. One cannot accept another’s responsibility in such things.