These verses contain two very strong warnings and we must understand what they mean.
The first warning in verse 18 is of not adding to the Word of God. What does Jesus mean when he says, "I warn everyone who hears the words of the prophecy of this book." The immediate context of this is clearly the book of Revelation, but I strongly believe that broadening the application to the whole of Scripture is appropriate. To add to this book is to have all the plagues mentioned in the book to be added to you. This is a warning that is used to keep people from tampering with this book.
- Deuteronomy 4:2: "You shall not add to the word that I command you, nor take from it, that you may keep the commandments of the LORD your God that I command you."
- Deuteronomy 12:32 "Everything that I command you, you shall be careful to do. You shall not add to it or take from it."
- Galatians 1:6-7 "I am astonished that you are so quickly deserting him who called you in the grace of Christ and are turning to a different gospel—not that there is another one, but there are some who trouble you and want to distort the gospel of Christ."
The 2nd part of the warning says something a bit different. Add to the Word and plagues are added to you, subtract from the Word and blessings are subtracted from you (v.19). In other words, you will not get to share in the tree of life which means you get to share in the lake of fire. You will not get to delight in God in the holy city, but rather you will be tormented along with the dragon, the beast and false prophet.
If these verses are rightly understood then they will serve as both a warning and words of encouragement. How? No one who has surrendered to the Lamb of God would add to or subtract from Scripture. 1Timothy 4:16 says, "Keep a close watch on yourself and on the teaching. Persist in this, for by so doing you will save both yourself and your hearers." 2Timothy 4:3-4 says, "For the time is coming when people will not endure sound teaching, but having itching ears they will accumulate for themselves teachers to suit their own passions, and will turn away from listening to the truth and wander off into myths." It is only those who are not under the authority of the Word of God (so they are lost) would assault the Word of God by adding to it or subtracting from it which invites such sever judgment.
- Believers love the Word as it is, unbelievers hate the Word and change it.
- Believers obey the Word no matter of inconvenience , unbelievers disobey the Word because of convenience.
- Believers willingly receive the Word, unbelievers willingly reject the Word.