A: When you read this Scripture, an angel is talking to John who has fallen down before him to worship him. Since he is simply a messenger, he makes it clear that he deserves no worship and that like John, he has a testimony of Jesus; therefore, he should worship God for the testimony of Jesus is the spirit of prophecy.
Prophecy has a couple of meanings: that of foretelling future events and forthtelling, which is preaching, proclamations, exhortations, and warnings that relate to the present spiritual environment. The main source behind all prophecy is the Holy Spirit. We know that God brings forth things not by might or power but by His Spirit (Zechariah 4:6). We know that all Scripture has been given by the inspiration of God, and the inspiration of God is the Holy Spirit who moved on men (prophets) in order to bring forth the Word of God (1 Timothy 3:16-17; 2 Peter 1:20-21). It is the Spirit of God who endows men with the gift of prophecy (Romans 12:6; 1 Corinthians 12:7-11). 1 Thessalonians 5:19-20 instructs us not to quench the Spirit and not to despise prophesyings. In Ephesians 1:17, we see Paul is asking on behalf of the believers that God will give them the spirit of wisdom, revelation and knowledge of Jesus, and we know according to John 16:13, that it is the Holy Spirit who leads us into all truth about Jesus.
The Word of God is comprised of prophecies that identify the Savior, the Promised One, the King of kings and Lord of lords, thereby, establishing a record, a testimony of the One who came as the Promised One of God over 20 centuries ago, died as Savior, rose as Lord of both the living and dead, and is coming back as King, the Lord Jesus Christ. The angel is establishing that the declarations he has brought forth and the record, and prophecies that John has been given are inspired by the same Spirit and that it is prophecy of Jesus Christ. Remember, Revelation 1:1 tells us it is the revelation of Jesus Christ, and the prophecies it contains are about Jesus returning as the victorious King of kings and Judge over all.