John the Apostle
Revelation 1:2ESV·traditional attribution

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

Matthew Henry Presbyterian @wholebiblehenry

Here we have, I. What we may call the pedigree of this book. 1. It is the revelation of Jesus Christ. The whole Bible is so; for all revelation comes through Christ and all centres in him; and especially in these last days God has spoken to us by his Son, and concerning his Son.

Commenting on Revelation 1:1-2

John Gill Reformed Baptist @doctorgill

Who bore record of the word of God,.... Of the essential and eternal Word of God, his only begotten Son; as John the apostle did in his Gospel, and in his epistles, and also in this book; and which is a clear evidence of his being the writer of it, And of the testimony of Jesus Christ; that is, the Gospel, which testifies of the...

Albert Barnes Presbyterian @notesbybarnes

Verse 2. Who bare record of the word of God. Who bore witness to, or testified of (εμαρτυρησε) the word of God. He regarded himself merely as a witness of what he had seen, and claimed only to make a fair and faithful record of it. : "This is the disciple which testifieth (ομαρτυρων) of these things, and wrote these things." : "And he that saw it bare record"--μεμαρτυρηκε.