When John heard what was being given him, he recognized its source and identified it. He also understood that it was opening up what was to occur and giving him what was to happen in prophetic terms in the future and end times as Christ had done when He was with the disciples on earth. So when he said the the testimony of Jesus was the Spririt of Prophecy, John meant it literally as he knew it was Jesus giving him insight and telling him what was to come.
"When you find men questioning the testimonies, finding fault with them, and seeking to draw away the people from their influence, be assured that God is not at work through them. It is another spirit. Doubt and unbelief are cherished by those who do not walk circumspectly. They have a painful consciousness that their life will not abide the test of the Spirit of God, whether speaking through His Word or through the testimonies of His Spirit that would bring them to His Word. Instead of beginning with their own hearts, and coming into harmony with the pure principles of the gospel, they find fault, and condemn the very means that God has chosen to fit up a people to stand in the day of the Lord." 1SM 45-48
- Revelation 1:2
Who bare record of the word of God, and of the testimony of Jesus Christ, and of all things that he saw.
Revelation 1:1-3 (in Context) Revelation 1 (Whole Chapter) - Revelation 1:9
I John, who also am your brother, and companion in tribulation, and in the kingdom and patience of Jesus Christ, was in the isle that is called Patmos, for the word of God, and for the testimony of Jesus Christ.
Revelation 1:8-10 (in Context) Revelation 1 (Whole Chapter) - Revelation 12:17
And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.
Revelation 12:16-17 (in Context) Revelation 12 (Whole Chapter) - Revelation 19:10
And I fell at his feet to worship him. And he said unto me, See thou do it not: I am thy fellowservant, and of thy brethren that have the testimony of Jesus: worship God: for the testimony of Jesus is the spirit of prophecy.
"When you find men questioning the testimonies, finding fault with them, and seeking to draw away the people from their influence, be assured that God is not at work through them. It is another spirit. Doubt and unbelief are cherished by those who do not walk circumspectly. They have a painful consciousness that their life will not abide the test of the Spirit of God, whether speaking through His Word or through the testimonies of His Spirit that would bring them to His Word. Instead of beginning with their own hearts, and coming into harmony with the pure principles of the gospel, they find fault, and condemn the very means that God has chosen to fit up a people to stand in the day of the Lord." 1SM 45-48