Charles Spurgeon Commentary 1 John 5:6-7

"This is he that came by water and blood, [even] Jesus Christ; not with the water only, but with the water and with the blood. And it is the Spirit that beareth witness, because the Spirit is the truth." — 1 John 5:6-7 (ASV)

This is he that came by water and blood, even Jesus Christ, not by water only, but by water and blood. And it is the Spirit that beareth witness, because the Spirit is truth. For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.

Thus all the Persons in the blessed Trinity confirm the faith of the Christian; the Father, the Son, and the Holy Ghost bear united witness to the faith which God himself gives us.

And it is the Spirit that bears witness, because the Spirit is truth.

For there are three that bear record in heaven—or, "witness in heaven."