John Calvin Commentary 1 John 5:7

"And it is the Spirit that beareth witness, because the Spirit is the truth." — 1 John 5:7 (ASV)

There are three that bear record in heaven. This entire verse has been omitted by some. Jerome thinks that this has happened by design rather than by mistake, and indeed only on the part of the Latins. But since even the Greek copies do not agree, I dare not assert anything on the subject. However, since the passage flows better when this clause is added, and as I see that it is found in the best and most approved copies, I am inclined to accept it as the true reading. And the meaning would be that God, in order to confirm our faith in Christ most abundantly, testifies in three ways that we ought to rest in Him. For as our faith acknowledges three persons in the one divine essence, so it is called in so real ways to Christ that it may rest on Him.

When He says, These three are one, He refers not to essence but, on the contrary, to consent; as if He had said that the Father and His eternal Word and Spirit harmoniously testify the same thing respecting Christ. Therefore, some copies have εἰς ἓν, “for one.” But though you read ἓν εἰσιν, as in other copies, yet there is no doubt that the Father, the Word and the Spirit are said to be one, in the same sense that afterwards the blood and the water and the Spirit are said to agree in one.

But since the Spirit, who is one witness, is mentioned twice, it seems to be an unnecessary repetition. To this I reply that since He testifies of Christ in various ways, a twofold testimony is fittingly ascribed to Him. For the Father, together with His eternal Wisdom and Spirit, declares Jesus to be the Christ authoritatively, as it were; then, in this case, the sole majesty of the Deity is to be considered by us. But since the Spirit, dwelling in our hearts, is an earnest, a pledge, and a seal, to confirm that decree, He thus speaks again on earth by His grace.

But since not all accept this reading, I will therefore expound what follows as if the Apostle referred to the witnesses only on the earth.