1 John 5:7 gives full proof of the plurality of Persons in the Godhead. "For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost; and these three are one."
This is not only a proof of the Deity of each of these three persons, inasmuch as they, are not only said to be "one", that is, one God; and their witness is called the witness of God (1 John 5:9), but of a Trinity of Persons, in the unity of the divine essence. This unity of essence, or nature, is asserted and secured, by their being said to be one. This respects not a mere unity of testimony, but of nature.
It is not said of them, as of the witnesses on earth, that they "agree in one"; but that they "are one". And they may be called a Trinity, inasmuch as they are "three"; and a Trinity of Persons, since they are not only spoken of as distinct from each other, the Father from the Word and Holy Ghost, the Word from the Father and the Holy Ghost, and the Holy Ghost from the Father and the Word; but a personal action is ascribed to each of them.
They are all three said to be testifiers, or to bear record; which cannot be said of mere names and characters; nor be understood of one person under different names. If the one living and true God only bears record, first under the character of a Father, then under the character of a Son, or the Word, and then under the character of the Holy Ghost; testimony, indeed, would be given three times, but there would be but one testifier, not three, as the apostle asserts.
Suppose one man should, for one man may bear the characters, and stand in the relations of father, son, and master; of a father to a child of his own; of a son, his father being living; and of a master to servants under him; suppose, I say, this man should come into a court of judicature, and be admitted to bear testimony in an affair there depending, and should give his testimony first under the character of a father, then under the character of a son, and next under the character of a master; every one will conclude, that though there was a testimony three times given, yet there was but one, and not three, that bore record.
This text is so glaring a proof of the doctrine of the Trinity, that the enemies of it have done all they can to weaken its authority, and have pushed hard to extirpate it from a place in the sacred writings. They object, that it is wanting in the Syriac version; that the old Latin interpreter has it not; that it is not to be found in many Greek manuscripts; and is not quoted by the ancient fathers who wrote against the Arians, when it might have been of great service to them.
To all which it may be replied; that as to the Syriac version, though an ancient one, it is but a version, and till of late appeared a very defective one. The history of the adulterous woman in the eighth of John, the second epistle of Peter, the second and third epistles of John, the epistle of Jude, and the book of Revelation, were all wanting, till restored from a copy of archbishop Usher's, by De Dieu and Dr. Pocock; and who also, from an Eastern copy, has supplied the version with this text, so that now it stands in it.
As to the old Latin interpreter, it is certain that it is to be seen in many Latin manuscripts of an early date, and is in the Vulgate Latin version of the London Polyglot Bible. The Latin translation which bears the name of Jerom has it; and who, in an epistle to Eustochium, prefixed to his translation of those canonical epistles, complains of the omission of it, by unfaithful interpreters.
As to its being wanting in some Greek manuscripts, it need only be said, it is found in many others. It is in the Complutensian edition, the compilers of which made use of various copies. Out of sixteen ancient copies of Robert Stephens's, nine of them had it. It is also said to be in an old British copy.
As to its not being quoted by some of the ancient fathers, this can be no proof of its not being genuine; since it might be in the original copy, and not in that used by them, through the carelessness and unfaithfulness of transcribers; or through copies erased falling into their hands, such as had been corrupted before the times of Arius, even by Artemon, or his disciples, who lived in the second century; who held that Christ was a mere man; by whom it is said, this passage was erased.
And certain it is, that this epistle was very early corrupted; as the ancient writers testify: It might be in the copies used by the fathers, and yet not quoted by them, having scriptures not without it, to prove and defend the doctrine of it. Yet, after all, it appears plainly to be quoted by many of them.
So that it is to be traced up within a hundred years, or less, the writing of the epistle. That should be sufficient to satisfy anyone of the genuineness of this text.
Besides, it should be observed, that there never was any dispute about it, until Erasmus left it out in the first edition of his translation of the New Testament. Yet he himself, upon the credit of the old British copy, before mentioned, put it into another edition of his translation. Yea, even the Socinians themselves have not dared to leave it out in their German Racovian version, A. C. 1630.
To which may be added, that the context requires it; the connection with the preceding verse shows it, as well as its opposition to, and distinction from, the following verse. And in 1 John 5:9 there is a plain reference to the divine witnesses in this verse. The inference in verse 9 would not be clear, if there was no mention before made of a divine testimony.
I do not rest the proof of the doctrine of the Trinity on this single passage; but on the whole current and universal consent of scripture, where it is written as with a sunbeam; according to which, a Trinity of Persons in the Godhead appears in the works of creation, providence, and grace; in all things respecting the office and work of Christ; in God's acts of grace towards and upon his people; and in their worship and duties of religion enjoined them, and practised by them.
This article was taken from Dr. Gill's A Body of Doctrinal and Practical Divinity.