In the First Epistle of John, Chap. V., vs. 7, most but not all Copies of the Latin Bible, called the Vulgate, read as follows:---
"For there are three who bear witness in heaven: the
Father, the Word, and the Holy Spirit; and these three
are one. And there are three that bear witness on
earth: the Spirit and the water and the blood: and
these three are one."
In the first printed edition of the New Testament,
called the Complutensian, prepared at Alcala in Spain
in 1514 by Cardinal Francis Ximenes, the words here
italicised were included, having been translated from
the Latin text into Greek; for the Greek MSS used did
not contain them. They are only found in two Greek
MSS., one of the fifteenth the other of the sixteenth
century. About 400 other Greek Codices from the fourth
century down to the fourteenth ignore them. All MSS of
the Old Latin Version anterior to Jerome lack them,
and in the oldest Copies even of Jerome's recension of
the Latin text, called the Vulgate, they are
conspicuously absent. The first Church writer to cite
the verse in such a text was Priscillian, a Spaniard,
who was also the first heretic to be burned alive by
the Church in the year 385. After him Vigilius, Bishop
of Thapsa, cites it about 484. It is probable that the
later Latin fathers mistook what was only a comment of
Cyprian Bishop of Carthage (died 258) for a citation
of the text. In any case, it filtered from them into
the Vulgate text,  from which, as we have seen, it
was translated into Greek and inserted in two or three
very late manuscripts.
Erasmus's first edition of the Greek Testament, in
1516, omitted the verse, as also did the second; but
in 1522 he issued a third edition containing it.
Robert Stephens also inserted it in his edition of
1546, which formed the basis of all subsequent
editions of the Greek Testament until recently, and is
known as the Received Text, or Textus Receptus.
See Conybeare's History of NT Criticism, pages 91-98.