John Calvin 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)

For this is the witness, or testimony, of God. The particle ὅτι does not indicate cause here, but should be understood as explanatory; for the Apostle, after reminding us that God deserves to be believed much more than men, now adds that we can have no faith in God except by believing in Christ, because God presents him alone to us and establishes us in him. He therefore infers that we believe safely and with tranquil minds in Christ, because God by his authority warrants our faith. He does not say that God speaks outwardly, but that every godly person feels within that God is the author of his faith. It therefore appears how different from faith is a fading opinion dependent on something else.

If we receive the witness, or testimony, of men, He proves, reasoning from the less to the greater, how ungrateful men are when they reject Christ, who has been approved by God, as he has related; for if in worldly affairs we rely on the words of men, who may lie and deceive, how unreasonable it is that God should be given less credit, when He is, as it were, sitting on His own throne, where He is the supreme judge.

Then our own corruption alone prevents us from receiving Christ, since He gives us full proof for believing in His power. Besides, He calls not only that which the Spirit imprints on our hearts the testimony of God, but also that which we derive from the water and the blood.

For that power of cleansing and expiating was not earthly, but heavenly. Hence the blood of Christ is not to be estimated according to the common manner of men; but we must rather look to the design of God, who ordained it for blotting out sins, and also to that divine efficacy which flows from it.