Who bore witness to the word of God, and to the testimony of Jesus Christ, to all things that he saw. Revelation 1:2
John is the one who bore testimony to the word of God. The book of Revelation is not a fairy tale or something made up. It is the word of God. The more we study God's word and open our heart to the leading of the Holy Spirit, the more we will come to understand God's word.
The vision John received concerning the book of Revelation is the testimony to who Jesus is. It reveals Him as the one who has authority, the one who will pour out God's wrath upon the earth, the one who will return with the armies of Heaven, and the one who will come to establish His kingdom. When we study and understand Revelation we gain a deeper understanding of who Jesus Christ truly is.