1:2who bore witness to the word of God and to the testimony of Jesus Christ, even to all that he saw.

who bore witness
    a) to the WORD of GOD and

This phrase is used so often...the angel is emphasizing that the book of Revelation is part of the Word of God. The "Word of God" even carries a prophetic tinge to it. Samuel, a prophet, told Saul to send his servant on ahead so that he might speak out the Word of God directly to him, Saul (1 Samuel 9:27). Similarly, John, too, stands us up alone (Revelation 19:10) and says, "Here comes the Word of God." This is serious ..... because in the next verse, we are to "heed" these words. Compare other instances of the use of 'the Word of God' and 'word of their testimony' of God:

Revelation 1:9
Revelation 6:9
Revelation 12:11
Revelation 17:17
Revelation 19:9

    b) to the testimony of Jesus Christ,

There is no doubt about it. This Revelation is a continuation of John's testimony of Jesus Christ. First, he closes his gospel, John 21:24-25, with his bearing witness to the things Jesus Christ did.
Then John continues in the beginning of his first letter to proclaim what he has seen and heard (1 John 1:1-4). Lastly, John shows his closeness to Jesus Christ with the Revelation of Jesus, Revelation 22:16.

    c) even to all that he saw.

John didn't leave anything out this time. He recorded everything meticulously, for us to read, hear, and heed. This is not a novel or science fiction book; it is what John actually saw (and heard) as he was transported divinely back to the future.