Commentary on the Comma Johanneum

1 John 5:7: “For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.”

This text (the Comma Johanneum) in no way demonstrates that the three persons are one individual or numeric essence. Nothing in the text even hints at anything remotely of that nature. Rather, the text is speaking about a unity of concord (agreement). It is a unity of accord, not a unity of metaphysical essence being referred to in this text. Even the most staunch trinitarians, such as John Calvin admit this in their commentary on the text:

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

Source: Calvin’s Commentary on the Bible

Samuel Clarke: Not one and the same person; but one and the same thing in effect; one and the same testimony. Though it ought not indeed to be concealed, that the whole passage here printed in a different character, since it has never yet been proved to be found in the text of any greek manuscript, before the invention of printing; nor in the text of any ancient version; nor was cited by any of the numerous writers in the whole Arian controversy; ought not to have much stress laid upon it in any question. And the sense of the Apostle is very complete without it, according to the following reading of all the greek MSS and ancient version: “Who is he that overcometh the world, but he that believeth that Jesus is the Son of God, This is he that came [that was declared and manifested to be the Son of God,] by water and blood, [by water, at his baptism, when there came a voice from heaven, saying, This is my beloved Son: and by blood, viz. by his death and resurrection:] – And it is the Spirit [the gifts of the Holy Ghost, and the power of miracles granted to the Apostles,] that bears witness; because the Spirit is truth. For there are three that bear record; the Spirit, the Water, and the Blood; and these three agree in one, [or, as some ancient writers read the text, these three are one, viz. one testimony, that Jesus is the Son of God.]

Source: The Scripture Doctrine of the Trinity