Calvin, Gill and Henry believed it should be there.
http://www.geocities.com/brandplucked/1John5-7.html
A Trail of Evidence
We find mention of 1 John 5:7, from about 200 AD through the 1500s. Here is a useful timeline of references to this verse:
Scholars often disagree with each other, but John Gill, in his well known commentary on the entire Bible, remarks concerning 1 John 5:7: "It is cited by Athanasius about the year 350 (Contra Arium p. 109); and before him by Cyprian in the middle of the "third" century, about the year 250 (De Unitate Eccles. p. 255. & in Ep. 73. ad Jubajan, p. 184.) and is referred to by Tertullian about, the year 200 (Contr. Praxeam, c. 25 ) 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."
200 AD - Tertullian's quote is debated, but he may well be referring to the phrase found only in 1 John 5:7 when he says: "And so the connection of the Father, and the Son, and of the Paraclete (Holy Ghost) makes three cohering entities, one cohering from the other, WHICH THREE ARE ONE entity, not one person. Just as it is said "I and the Father are one entity" refers to the unity of their substance, not to oneness of their number."
250 AD - Cyprian of Carthage, wrote, "And again, of the Father, Son, and Holy Ghost it is written: "And the three are One" in his On The Lapsed, On the Novatians. Note that Cyprian is quoting and says "IT IS WRITTEN, And the three are One." He lived from 180 to 250 A.D. and the scriptures he had at that time contained the verse in question. This is at least 100 years before anything we have today in the Greek copies. If it wasn't part of Holy Scripture, then where did he see it WRITTEN?
350 AD - Priscillian referred to it [Corpus Scriptorum Ecclesiasticorum Latinorum, Academia Litterarum Vindobonensis, vol. xviii, p. 6.]
350 AD - Idacius Clarus referred to it [Patrilogiae Cursus Completus, Series Latina by Migne, vol. 62, col. 359.]
350 AD - Athanasius referred to it in his De Incarnatione
380 AD - Priscillian in Liber Apologeticus quotes "and there are three which give testimony in heaven, the Father, the Word, and the Spirit, and these three are one in Christ Jesus."
Likewise, the anti-Arian work compiled by an unknown writer, the Varimadum (380 AD) states: "And John the Evangelist says, . . . “And there are three who give testimony in heaven, the Father, the Word, and the Spirit, and these three are one”. (Varimadum 90:20-21).
398 AD - Aurelius Augustine used it to defend Trinitarianism in De Trinitate against the heresy of Sabellianism
415 AD - Council of Carthage. The contested verse (1 John 5:7) is quoted at the Council of Carthage (415 A. D.) by Eugenius, who drew up the confession of faith for the "orthodox." It reads with the King James. How did 350 prelates in 415 A.D. take a verse to be orthodox that wasn't in the Bible? It had to exist there from the beginning. It was quoted as "Pater, VERBUM, et Spiritus Sanctus".
450-530 AD. Several orthodox African writers quoted the verse when defending the doctrine of the Trinity against the gainsaying of the Vandals. These writers are:
A) Vigilius Tapensis in "Three Witnesses in Heaven"
B) Victor Vitensis in his Historia persecutionis [Corpus Scriptorum Ecclesiasticorum Latinorum, Academia Litterarum Vindobonensis, vol. vii, p. 60.]
C) Fulgentius in "The Three Heavenly Witnesses" [Patrilogiae Cursus Completus, Series Latina by Migne, vol. 65, col. 500.]
500 AD - Cassiodorus cited it [Patrilogiae Cursus Completus, Series Latina by Migne, vol. 70, col. 1373.]
527 AD - Fulgentius in Contra Arianos stated: "Tres sunt qui testimonium perhibent in caelo. Pater, Verbum et Spiritus, et tres unum sunt."
550 AD - The "Speculum" has it [The Speculum is a treatise that contains some good Old Latin scriptures.]
636 AD - Isidor of Seville quotes the verse as it stands in the KJB.
750 AD - Wianburgensis referred to it
800 AD - Jerome's Vulgate has it [It was not in Jerome's original Vulgate, but was brought in about 800 AD from good Old Latin manuscripts.] It is also in the Clementine Vulgate today.
157-1400 AD. Waldensian (that is, Vaudois) Bibles have the verse.
Now the "Waldensian," or "Vaudois" Bibles stretch from about 157 to the 1400s A.D. The fact is, according to John Calvin's successor Theodore Beza, that the Vaudois received the Scriptures from missionaries of Antioch of Syria in the 120s A.D. and finished translating it into their Latin language by 157 AD. This Bible was passed down from generation, until the Reformation of the 1500s, when the Protestants translated the Vaudois Bible into French, Italian, etc. This Bible carries heavy weight when finding out what God really said. Theodore Beza, John Wesley and Johnathan Edwards believed, as most of the Reformers, that the Vaudois were the descendants of the true Christians, and that they preserved the Christian faith for the Bible-believing Christians today.