The phrase "the testimony of Jesus" is mentioned four times in the book of Revelation; the context in which it is used makes the meaning plain. In Revelation 1:2, it refers back to the first verse, which says that God gave Jesus Christ this special message; and Christ in turn sent it to John by an angel. In other words, what we now have as the book of Revelation is "the testimony of Jesus." The ninth verse of the first chapter uses the phrase in the same way.
Revelation 12:17 says that the true Church has this testimony and keeps the commandments of God, meaning the same as Christ said in John 15:14, "You are My friends if you do whatever I command you." The Church of God lives by Christ's instructions. The commandments of God and Christ's instructions are one and the same.
The last reference is in Revelation 19:10, where the angel quickly restrains John from worshipping him. Instead, the angel said, "Worship God! For the testimony of Jesus is the spirit of prophecy."
Some attempt to draw a different doctrine from this one verse. But if we allow the Bible to interpret itself, we see that the testimony of Jesus is the words of Christ in general and the revelation of this book in particular. "Prophecy" can mean foretelling the future or it can mean inspired preaching. Both meanings fit here. In other words, the angel was saying: "Don't worship me! You should only worship God! And your focus should be on preaching the word Jesus told me to give to you."
For more understanding, please read our booklet The Book of Revelation Unvailed.