Bare witness (εμαρτυρησεν). First aorist active indicative of μαρτυρεω, which, along with μαρτυς and μαρτυρια, is common in all the Johannine books (cf. Revelation 22:18; Revelation 22:20), usually with περ or οτ, but with cognate accusative as here in Revelation 22:16; Revelation 22:20; 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 Revelation 1:9; Revelation 6:9; Revelation 20:4, not the personal Word as in Revelation 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.