Jamieson, Fausset and Brown on 1 John 5:9

9. If, &c.--We do accept (and rightly so) the witness of veracious men, fallible though they be; much more ought we to accept the infallible witness of God (the Father). "The testimony of the Father is, as it were, the basis of the testimony of the Word and of the Holy Spirit; just as the testimony of the Spirit is, as it were, the basis of the testimony of the water and the blood" [Bengel].

for--This principle applies in the present case, FOR, &c.

which--in the oldest manuscripts, "because He hath given testimony concerning His Son." What that testimony is we find above in 1Jo 5:1, 5, "Jesus is the Christ, the Son of God"; and below in 1Jo 5:10, 11.