If we receive the witness of men, the witness of God is greater: for this is the witness of God which he hath testified of his Son.
If ... We do accept (and rightly) 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 the basis of that of the Word and of the Holy Spirit; just as the testimony of the Spirit is the basis of the testimony of the water and the blood' (Bengel).
For. This principle applies in the present case, FOR, etc.
Which. In 'Aleph (') A B, Vulgate, 'Because He hath given testimony concerning His Son.' What that testimony is, we find in 1 John 5:1; 1 John 5:5; 1 John 5:10-11.