{9} 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:

(9) The supplication of John (which is the third part of the confirmation) joined with a curse of abhorrence, to preserve the truth of this book entire and uncorrupted in two verses.

People's New Testament

22:18 If any man shall add unto these things, God shall add unto him the plagues that are written in this book. See De 4:2 12:32 The whole spirit of the Scripture is against adding to or taking from the Lord's words. This is a warning against spurious revelations.

Wesley's Notes

22:18-19 I testify to every one, and c. - From the fulness of his heart, the apostle utters this testimony, this weighty admonition, not only to the churches of Asia, but to all who should ever hear this book. He that adds, all the plagues shall be added to him; he that takes from it, all the blessings shall be taken from him; and, doubtless, this guilt is incurred by all those who lay hinderances in the way of the faithful, which prevent them from hearing their Lord's I come, and answering, Come, Lord Jesus. This may likewise be considered as an awful sanction, given to the whole New Testament; in like manner as Moses guarded the law, Deut 4:2, and Deut 12:32; and as God himself did, Mal 4:4, in closing the canon of the Old Testament.

Jamieson-Fausset-Brown Bible Commentary

18. For I testify-None of our manuscripts have this. A, B, Vulgate, and Andreas read, "I" emphatic in the Greek. "I testify."

unto these things-A, B, and Andreas read, "unto them."

add . add-just retribution in kind.

Matthew Henry's Concise Commentary

22:6-19 The Lord Jesus spake by the angel, solemnly confirming the contents of this book, particularly of this last vision. He is the Lord God faithful and true. Also by his messengers; the holy angels showed them to holy men of God. They are things that must shortly be done; Christ will come quickly, and put all things out of doubt. And by the integrity of that angel who had been the apostle's interpreter. He refused to accept religious worship from John, and reproved him for offering it. This presents another testimony against idolatrous worship of saints and angels. God calls every one to witness to the declarations here made. This book, thus kept open, will have effect upon men; the filthy and unjust will be more so, but it will confirm, strengthen, and further sanctify those who are upright with God. Never let us think that a dead or disobedient faith will save us, for the First and the Last has declared that those alone are blessed who do his commandments. It is a book that shuts out form heaven all wicked and unrighteous persons, particularly those who love and make lies, therefore cannot itself be a lie. There is no middle place or condition. Jesus, who is the Spirit of prophecy, has given his churches this morning-light of prophecy, to assure them of the light of the perfect day approaching. All is confirmed by an open and general invitation to mankind, to come and partake freely of the promises and of the privileges of the gospel. The Spirit, by the sacred word, and by convictions and influence in the sinner's conscience, says, Come to Christ for salvation; and the bride, or the whole church, on earth and in heaven, says, Come and share our happiness. Lest any should hesitate, it is added, Let whosoever will, or, is willing, come and take of the water of life freely. May every one who hears or reads these words, desire at once to accept the gracious invitation. All are condemned who should dare to corrupt or change the word of God, either by adding to it, or taking from it.