The indicative mood, assuming such reception as a fact. If we receive, as we do. On the verb receive, see on Joh 3:32.
The witness of God is greater
Supply mentally, and therefore we should receive that.
For (ὅτι)
Not explaining why it is greater, but why the principle of the superior greatness of divine testimony should apply and be appealed to in this case. Supply mentally, and this applies in the case before us, for, etc.
This is the witness of God which (ἣν)
The best texts read ὅτι that or because. Render that. This is the witness of God, even the fact that, etc.