The following is an excerpt from The MacArthur New Testament Commentary on Revelation 22.

I testify to everyone who hears the words of the prophecy of this book: if anyone adds to them, God will add to him the plagues which are written in this book; and if anyone takes away from the words of the book of this prophecy, God will take away his part from the tree of life and from the holy city, which are written in this book. (22:18–19)It is of great significance that the Bible closes with an affirmation of its truthfulness. Because the words of Scripture are “faithful and true” (22:6), they must not be sealed up, but proclaimed (22:10). Sinners are to be called to respond to the warnings in the Word of the living God or suffer the consequences. All the prophecies of Revelation regarding the doom of sinners will come true. That terrifying certainty should drive people to Jesus Christ to escape the wrath to come (1 Thess. 1:10).

The speaker who testifies to the authority and finality of the words of the prophecy of this book is none other than the Lord Jesus Christ (cf. v. 20). His solemn warning against tampering with Scripture applies first of all to the prophecy of the book of Revelation (cf. 1:3). Its stern rebukes of Jezebel and her followers (2:20–23), those who had embraced the “deep things of Satan” (2:24), and those of the “synagogue of Satan” (3:9) would have prompted them to assault it. Down through the centuries there have been others who have both attacked Revelation and seriously misinterpreted it. But in light of the repeated warnings against altering God’s Word, Christ’s warning must be broadened to include all of Scripture. In Deuteronomy 4:2 Moses cautioned, “You shall not add to the word which I am commanding you, nor take away from it, that you may keep the commandments of the Lord your God which I command you.” In Deuteronomy 12:32 he added, “Whatever I command you, you shall be careful to do; you shall not add to nor take away from it.” Proverbs 30:5–6 warns, “Every word of God is tested; He is a shield to those who take refuge in Him. Do not add to His words or He will reprove you, and you will be proved a liar.” Thus, the prohibition against altering the Apocalypse by implication extends to all of Scripture. Because Revelation describes the entire sweep of history from the close of the apostolic age to the eternal state, any alteration of it would be an alteration of Scripture, as Robert L. Thomas notes:  The predictive portions project from John’s lifetime all the way into the eternal state. Any type of prophetic utterance would intrude into the domain of this coverage and constitute either an addition to or subtraction from Revelation’s content. So the final book of the Bible is also the concluding product of NT prophecy. It also marks the close of the NT canon since the prophetic gift was the divinely chosen means for communicating the inspired books of the canon. (Revelation 8–22: An Exegetical Commentary [Chicago: Moody, 1995], 517)The canon of Scripture was closed at the end of the first century when Revelation was finished. Thus, any false prophet, fraud, or charlatan who adds alleged new revelations to it (as the Montanists did in the early church and Joseph Smith, Mary Baker Eddy, and other false prophets have done in recent times) will face divine vengeance. God will add to such people the plagues which are written in the book of Revelation. God’s judgment will be equally severe on anyone who takes away from the words of Scripture (as the heretic Marcion did in the early church and liberal higher critics have done in modern times)—God will take away their part from the tree of life and from the holy city. Both warnings contain a play on words. Those who add to Scripture will have plagues added to them; those who take away from Scripture will have the blessings of heaven taken away from them.

No true believer would ever deliberately tamper with Scripture. Those who know and love God will treat His Word with the utmost respect. They will say with the psalmist, “O how I love Your law!” (Ps. 119:97; cf. Pss. 119:113, 163, 167; John 14:23); and, “I delight in Your law” (Ps. 119:70; cf. Pss. 1:2; 119:77, 92, 174). That does not, of course, mean that believers will never make errors in judgment or mistakenly interpret Scripture incorrectly or inadequately. The Lord’s warning here is addressed to those who engage in deliberate falsification or misinterpretation of Scripture, those whom Paul denounces as peddlers of the Word of God (2 Cor. 2:17).

At the conclusion of his commentary on Revelation, J. A. Seiss expressed the humble reverence for Scripture that marks true believers:  O, my friends, it is a fearful thing to suppress or stultify the word of God, and above all “the words of the prophecy of this Book.” To put forth for truth what is not the truth,—denounce as error, condemn, repudiate, or emasculate what God himself hath set his seal to as his mind and purpose, is one of those high crimes, not only against God, but against the souls of men, which cannot go unpunished. With an honest and ever-prayerful heart, and with these solemn and awful warnings ever before my eyes, I have endeavoured to ascertain and indicate in these Lectures what our gracious Lord and Master has been so particular to make known and defend. If I have read into this Book anything which he has not put there, or read out of it anything which he has put there, with the profoundest sorrow would I recant, and willingly burn up the books in which such mischievous wickedness is contained. If I have in anything gone beyond the limits of due subjection to what is written, or curtailed in any way the depth and measure of what Jesus by his angel has signified for the learning of the Churches, I need not the condemnation of men to heap upon me the burden of censure which I deserve. If feebleness, or rashness, or overweening confidence in my own understanding has distorted anything, I can only deplore the fault, and pray God to send a man more competent to unfold to us the mighty truths which here stand written. According to the grace and light given me, have I spoken.… If I err, God forgive me! If I am right, God bless my feeble testimony! In either case, God speed his everlasting truth! (The Apocalypse [reprint, Grand Rapids: Kregel, 1987], 527)Revelation and the rest of Scripture are true, and the redeemed will believe the Bible, guard the Bible, love the Bible, and obey the Bible. That Scripture speaks truly when it describes the joys of heaven and the terrors of hell should motivate sinners to heed God’s gracious call to salvation.