en_tn_condensed/rev/22/18.md

General Information:

John gives his final remarks about the book of Revelation.

I testify

Here "I" refers to John.

the words of the prophecy of this book

Here "words" refers to the message that they formed. See how you translated this in Revelation 22:7. Alternate translation: "This prophetic message of this book" (See: rc://en/ta/man/translate/figs-metonymy)

If anyone adds to them ... If anyone takes away

This is a strong warning to not change anything about this prophecy.

that are written about in this book

This can be stated in active form. Alternate translation: "which I have written about in this book" (See: rc://en/ta/man/translate/figs-activepassive)