"For there are three which bear record in heaven, the Father, the Word, and the holy Ghost: and these three are one." -Geneva Bible
This verse, technically referred to as "the Johannine Comma," has been controversial for a long time. However, it is important to recognize that orthodox Christians are unanimous in holding to the substance of the teaching of the verse. Most modern Bible translations, including my preferred ESV, leave it out. Older ones, such as the Geneva Bible and KJV include it. That is because the editors of the critical versions of the Greek New Testament have concluded that it isn't supported by the manuscripts. I am not qualified to address that. The following is from the Systematic Theology of John Brown of Haddington. He was a mid-18th Century theology professor for the Burgher Associate Synod in Scotland. I reprint it here simply as a contribution to the discussion.
"The Socinians, modern Arians, and some others, contend that the last-mentioned text, I John v. 7, is spurious; because, 1. 'Many Greek manuscripts want [lack] it.' But many of these want other texts: and the similarity of the 7th and 8th verses made a careless transcriber apt to overleap one of them. 2. 'Many of the ancient translations want it.' But none of these translations are of great weight in this matter, for they want much more of the New Testament. Nor are any of them, except the Syriac and Jerome's Latin one, much worth. 3. 'The ancient Fathers do not quote it, when, in their disputes with heretics, it would have been much to their purpose.' But that might be because they had deficient copies, or cared not to adduce a text which their opponents might have rejected. -Let it be further observed, 1, The orthodox had no temptation to forge it, having plenty of proof for their faith concerning the Trinity beside. But the Antitrinitarians had strong temptations to drop it out of their copies, which is also more easily done. And yet perhaps it originated from no design, but from the hurry of a transcriber, amidst the rage of persecution. 2. About 1400 years ago [i.e., before Brown's time], we find complaints of some Antitrinitarians attempting to corrupt the Scripture; but never, till of late, that the orthodox had done so. 3. This verse is referred to by Tertullian about AD 200, quoted by Cyprian about 250, and by Athanasius, or one in his name, about 350. Jerome hath it in his translation about 400, and admitting it to be in all the best Greek copies, he severely blames the want of it in the old Latin version. Soon after, it is quoted by Eucherus and Vigilius. In 484, the African bishops quote it in the Confession of their faith which they presented to Hunneric their Arian king; and about thirty years after, Fulgentius, when required by an Arian king to produce his objections against the Arians, quoted it three times. When the Vulgate Latin translation was solemnly, and with great care, corrected from Greek and Latin manuscripts, by order of Charles the Great, about AD 800, and again by the famed University of Sorbonne, about two hundred years after, this text was retained. Erasmus, who inclined to Arianism, first suspected it, and dropt it out of his first edition of the New Testament: but restored it in his subsequent editions, upon the credit of an old British copy. It is said that nine of Stephen's sixteen manuscripts from which he printed his excellent edition of the Greek New Testament, had this text. No doubt, many of the manuscripts, from which other principal editions were formed, are now lost. A printed copy is even more authentic than almost any manuscript extant, the oldest of which were written some hundred years after all these of the apostles were either worn out, or lost: for, more learning and care have been exercised to render some printed editions correct, than perhaps was taken on all the manuscripts written for a thousand years before the Reformation. 4. The passage appears deficient and unconnected if this verse be dropt. Mill and Bengelius have therefore honestly retained it, in their excellent editions, notwithstanding they have fairly. and with much more candour than Michaelis, represented the objections against it."
In contrast, Free Church of Scotland Theologian William Cunningham, 1805-1861, stated in his Historical Theology (1862, Vol. 2, p. 216), "most Trinitarians now admit that there is a decided preponderance of critical evidence against the genuineness of I John v. 7, usually spoken of as the three heavenly witnesses." This sentence appears in the section in which Cunningham sets forth the errors of the Socinians and Arians on the Trinity and divinity of Christ.