The entire reading was included in the earlier Catholic bibles like the 1582 Douay-Rheims and as late as the Douay version of 1950, but removed from later Catholic versions (St. Joseph NAB 1969, New Jerusalem bible 1985), but now once again the 2009 The Sacred Bible Public Domain Version has gone back to include it!
Here is just a partial list of those who contended for the authenticity of this verse.
Cyprian - 250 AD, Athanasius 350 A.D., Priscillian -385 AD, Jerome 420 AD, Fulgentius (late 5th century), Cassiodorus, Isidore of Seville, Jaqub of Edessa, Thomas Aquinas, John Wycliffe, Desiderus Erasmus, Stephanus, Lopez de Zuniga, John Calvin, Theodore Beza, Cipriano de Valera, John Owen, Francis Turretin, John Wesley, John Gill, Matthew Henry, Andrew Fuller, Thomas F. Middleton, Luis Gaussen, Frederick Nolan, Robert L. Dabney, Thomas Strouse, Floyd Jones, Peter Ruckman, George Ricker Berry, Edward F. Hills, David Otis Fuller, Thomas Holland, Michael Maynard and Donald A. Waite.
Those that deny 1John 5:7-8 must not understand it.
John was writing of what he personally witnessed at the cross.
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But what is missing is the verse titling NT pastors "priests," and any believer addressing created beings in prayer to Heaven, and a promise of Mary being bodily assumed into Heaven and crowned before the Lord returns, and of any apostolic successor for James, (Acts 12:1,2) as for Judas, which was to maintain the original 12 before the church formally began, and of an infallible magisterium being essential or promised, or of a clear example of an infant being baptized, etc .
Thus the efficacy of forgeries in R. Catholic history.
Thanks, interesting.