John Calvin Commentary 1 John 5:8

"For there are three who bear witness, the Spirit, and the water, and the blood: and the three agree in one." — 1 John 5:8 (ASV)

There are three He applies what had been said of water and blood to its own purpose, so that those who reject Christ might have no excuse. For by testimonies abundantly strong and clear, he proves that it is he who had been formerly promised, since water and blood, being the pledges and the effects of salvation, really testify that he had been sent by God.

He adds a third witness, the Holy Spirit, who nevertheless holds the first place, for without him the water and blood would have flowed without any benefit. For it is he who seals on our hearts the testimony of the water and blood; it is he who by his power makes the fruit of Christ’s death reach us; indeed, he makes the blood shed for our redemption penetrate our hearts, or, to say all in one word, he makes Christ with all his blessings become ours.

So Paul, in Romans 1:4, after having said that Christ by his resurrection manifested himself to be the Son of God, immediately adds, Through the sanctification of the Spirit. For whatever signs of divine glory may shine forth in Christ, they would still be obscure to us and escape our vision, if the Holy Spirit did not open for us the eyes of faith.

Readers may now understand why John presented the Spirit as a witness together with the water and the blood, specifically because it is the special office of the Spirit to cleanse our consciences by the blood of Christ, and to make the cleansing brought about by it effective. On this subject, some remarks are made at the beginning of the Second Epistle of Peter, where he uses nearly the same way of speaking, that is, that the Holy Spirit cleanses our hearts by the sprinkling of the blood of Christ.

But from these words we may learn that faith does not lay hold of a bare or an empty Christ, but that his power is at the same time vivifying. For to what purpose has Christ been sent to the earth, except to reconcile God by the sacrifice of his death, and except also that the office of washing had been allotted to him by the Father?

It may, however, be objected that the distinction mentioned here is superfluous because Christ cleansed us by expiating our sins; in that case, the Apostle mentions the same thing twice. I indeed allow that cleansing is included in expiation; therefore, I made no difference between the water and the blood, as though they were distinct. But if any one of us considers his own infirmity, he will readily acknowledge that it is not in vain or without reason that blood is distinguished from the water.

Besides, the Apostle, as it has been stated, alludes to the rites of the law; and God, on account of human infirmity, had formerly appointed not only sacrifices but also washings. And the Apostle meant distinctly to show that the reality of both has been exhibited in Christ, and for this reason he had said before, Not by water only, for he means that not only some part of our salvation is found in Christ, but the whole of it, so that nothing is to be sought elsewhere.