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 (Rev 22:18).
Question. Does Rev 22:18 prove people can lose eternal life?
Answer. Thanks for your question. Let me address two issues.
First, the “plagues” do not refer to hell. In the book of Revelation, the plagues are poured out upon people because of their sin. God judges them. They experience many negative things, including death. John is saying that if anybody adds to the words of God in the book, then he is in danger of experiencing God’s judgment in this life.
Second, the verb “will add” is different in the majority of manuscripts of Revelation. It is an aorist infinitive. That means it is more of a request that God may send these plagues.
What John is saying is that if someone adds to these words, John asks God to judge/discipline that person. John wants the reader to take the book of Revelation very seriously.