The phrase these things
refers to the revelation God had shown his servants via the chain of God-Christ-angel-John-servants (Revelation 1:1). By now testifying that he had actually heard and seen these things John sets himself up to share how he responded. That is significant because Revelation 22:7 had highlighted the need for a positive response.
8 I, John, am the one who heard and saw these things. And when I heard and saw them, I fell down to worship at the feet of the angel who showed them to me,