A.T. Robertson Commentary Revelation 1:2

"who bare witness of the word of God, and of the testimony of Jesus Christ, [even] of all things that he saw." — Revelation 1:2 (ASV)

Bare witness (εμαρτυρησεν). First aorist active indicative of μαρτυρεω, which, along with μαρτυς and μαρτυρια, is common in all the Johannine books (cf. 22:18,20), usually with περ or οτ, but with cognate accusative as here in 22:16,20; 1 John 5:10. Epistolary aorist here, referring to this book.

The word of God (τον λογον του θεου). Subjective genitive, given by God. The prophetic word as in 1:9; 6:9; 20:4, not the personal Word as in 19:14.

The testimony of Jesus Christ (την μαρτυριαν Ιησου Χριστου). Subjective genitive again, borne witness to by Jesus Christ.

Even of all the things that he saw (οσα ειδεν). Relative clause in apposition with λογον and μαρτυριαν.