Rev. Matthew Winzer:
Turretin’s Opera, vol. 4, pp. 289ff., contains the theological disputation on the Three Heavenly Witnesses. The respondent is Benedict Pictet.
Pictet’s view is expressed in his Christian Theology, p. 103: “There are, therefore, three persons in one divine essence; and this is clearly established by the passage in 1 John v. 7, which is brought forward and quoted by Cyprian, although not read in many copies. A far greater number of reasons can be alleged why this passage should be said to have been struck out by heretics, than to have been inserted by the orthodox. It was more to the advantage of heretics to suppress this passage, than to that of the orthodox to add it, because, if it were genuine, the heresy of the former would be entirely overthrown; if spurious, the orthodox creed was in no danger, being clearly established from other passages of scripture. The connection also of the text confirms our opinion; for unless this verse be admitted, there seems no reason why John should say, ‘There are three that bear witness in earth,’ not having before said any thing of ‘three witnesses in heaven.’ Nor can it be objected that these words in earth, were also added afterwards, for the contrary appears from verse 9, where mention is made both of the divine and the human testimony, ‘If we receive the witness of men, the witness of God is greater.'”
It will be observed that the number of copies is not relevant to its inclusion. It has early support in Cyprian, there is greater reason why it would have been suppressed than added, and internally the passage is coherent with its inclusion.
Source: https://puritanboard.com/threads/james-white-the-received-text.92697/page-2, Comment 31