# who testified about ... all the things that he saw

The word "who" refers to John. This tells what John did after he learned about the revelation. Alternate translation: "And John testified about ... all the things that he saw"

# the word of God

"the message that God spoke"

# the testimony of Jesus Christ

These words could refer to 1) the testimony that John has given about Jesus Christ. Alternate translation: "he has also given testimony about Jesus Christ" Or 2) the testimony that Jesus Christ has given about himself.