"For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one." 1 John 5:7
I am posting only to mainly quote Dr. John Gill regarding this passage as it is so very good and so very lost upon the modern Christian mind. Perhaps it would be pleasing to the Lord to have this little witness out there in support of this verse as the authentic Word of God which so many deny today.
This is such a clear and strong text of Scripture to support the doctrine of the Trinity none can deny. What they have denied is that this verse is Scripture at all and was inserted by a scribe or by Erasmus. Why we are so quick to accept such views without doing our own research is just a symptom of the age and times we find ourselves in. Consider Dr. Gill's commentary who wrote in the 18th century when all texts of the Greek scriptures and body of knowledge was to be had including the Alexandrian Greek texts which our modern translations make too much of. This portion simply deals with the authenticity of the text itself.
"For there are three that bear record in heaven,.... That is, that Jesus is the Son of God. The genuineness of this text has been called in question by some, because it is wanting in the Syriac version, as it also is in the Arabic and Ethiopic versions; and because the old Latin interpreter has it not; and it is not to be found in many Greek manuscripts; nor cited by many of the ancient fathers, even by such 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, which is the most ancient, and of the greatest consequence, it is but a version, and a 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 the Revelations, were formerly wanting in it, till restored from Bishop Usher's copy by De Dieu and Dr. Pocock, and who also, from an eastern copy, has supplied this version with this text. As to the old Latin interpreter, it is certain it is to be seen in many Latin manuscripts of an early date, and stands in the Vulgate Latin edition of the London Polyglot Bible: and the Latin translation, which bears the name of Jerom, has it, and who, in an epistle of his to Eustochium, prefixed to his translation of these canonical epistles, complains of the omission of it by unfaithful interpreters. And as to its being wanting in some Greek manuscripts, as the Alexandrian, and others, it need only be said, that it is to be found in many others; it is in an old British copy, and in the Complutensian edition, the compilers of which made use of various copies; and out of sixteen ancient copies of Robert Stephens's, nine of them had it: and as to its not being cited by some of the ancient fathers, this can be no sufficient proof of the spuriousness of it, since it might be in the original copy, though not in the copies used by them, through the carelessness or unfaithfulness of transcribers; or it might be in their copies, and yet not cited by them, they having Scriptures enough without it, to defend the doctrine of the Trinity, and the divinity of Christ: and yet, after all, certain it is, that it is cited by many of them; by Fulgentius (z), in the beginning of the "sixth" century, against the Arians, without any scruple or hesitation; and Jerom, as before observed, has it in his translation made in the latter end of the "fourth" century; and it is cited by Athanasius (a) about the year 350; and before him by Cyprian (b), in the middle, of the "third" century, about the year 250; and is referred to by Tertullian (c)about, the year 200; and which was within a "hundred" years, or little more, of the writing of the epistle; which may be enough to satisfy anyone of the genuineness of this passage; and besides, there never was any dispute about it till Erasmus left it out in the, first edition of his translation of the New Testament; and yet he himself, upon the credit of the old British copy before mentioned, put it into another edition of his translation."