Revelation 22:18-19
18 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:
19 And if any man shall take away from the words of the book of this prophecy, God shall take away his part out of the book of life, and out of the holy city, and from the things which are written in this book.
The Grace New Testament Commentary - Revelation 22:18-19
22:18-19 As the Book of Revelation closes, John forbids tampering with its text. He warns that anyone who adds to these things, God will add to him (MT: "may God add to him") the plagues that are written in this book; and if anyone takes away from the words of the book of this prophecy, God shall take away (MT: "may God take away") his part from the Book of Life (MT: "tree of life"), from the holy city, and from the things which are written in this book. As reflected in the Majority Text (MT), this is not so much a prediction as it is an imprecation (a wish of judgment and difficulty on another).
This is an expression of holy desire that the seriousness of these acts be met with severe divine judgment. John is not making an exact prediction about what will happen to people who add to or subtract from the Book of Revelation; the imprecations express judgments John believes such people deserve. He wants the retribution to be commensurate with the guilt. Throughout history, God has sent plagues on people who were disobedient to Him that resemble the plagues of the Tribulation (e.g., the plagues of Egypt). And as He sees fit, He can still bring similar troubles, diseases, and so forth, on those who deliberately revise Scripture today. He can "add to" and "take away" from them as He desires (including eternal rewards like the tree of life).
Charles Bing - Grace, Salvation, and Discipleship - Whoever Adds or Subtracts from God's Word, Revelation 22:18-19
(Salvation) Interpretation: Those who change God's Word will lose their salvation.
(Discipleship) Interpretation: Those who change God's Word lose eternal blessings.
The severe language of verse 18 reminds of the imprecatory wishes written by the psalmists of the Old Testament. Considering the nature of the plagues mentioned in Revelation, this could not be a literal threat. The author's purpose is to amplify the seriousness of changing God's truth.
Verse 19 in the New King James Version uses the word "Book" of life, but the overwhelming manuscript evidence supports the word "tree" of life, as seen in almost all of the other English translations. With that understood, we see that verse 19 applies to those who are believers because only they have any part in the "tree of life" and the "holy city" of the New Jerusalem. Also, it would be primarily believers who read, teach, and handle the book of Revelation. The threat is not that God will take away salvation from those who tamper with these holy words, but that He will deny special privileges to the offenders.
The warning certainly does not address those who arrive at various interpretations of the Bible which may be wrong. It is more a powerful warning not to manipulate or misuse God's revelation with malfeasance. Thus, the importance of good Bible study with pure hearts.