A.T. Robertson Commentary 1 John 5:9

"If we receive the witness of men, the witness of God is greater: for the witness of God is this, that he hath borne witness concerning his Son." — 1 John 5:9 (ASV)

If we receive (ε λαμβανομεν). Condition of first class with ε and the present active indicative, assumed as true. The conditions for a legally valid witness are laid down in De 19:15 (cf. Matthew 18:16; John 8:17f.; 10:25; 2 Corinthians 13:1).

Greater (μειζων). Comparative of μεγας, because God is always true.

For (οτ). So it applies to this case.

That (οτ). Thus taken in the declarative sense (the fact that) as in Joh 3:19, though it can be causal (because) or indefinite relative with μεμαρτυρηκεν (what he hath testified, perfect active indicative of μαρτυρεω, as in Joh 1:32; 4:44, etc.), a harsh construction here because of μαρτυρια, though some MSS. do read εν to agree with it (cf. verse 10). See οτ εαν in 3:20 for that idiom. Westcott notes the Trinity in verses 6-9: the Son comes, the Spirit witnesses, the Father has witnessed.