Snake handling

I guess those before the Middle Ages didn't have the complete "preserved Word of God".

These extra words are generally absent from the Greek manuscripts. In fact, they only appear in the text of four late medieval manuscripts. They seem to have originated as a marginal note added to certain Latin manuscripts during the middle ages, which was eventually incorporated into the text of most of the later Vulgate manuscripts.

 

I'm glad you posted that. From John Gill's commentary:

"The genuineness of this text has been called in question by some, because it is wanting in the Syriac version, as it also is in the Arabic and Ethiopic versions; and because the old Latin interpreter has it not; and it is not to be found in many Greek manuscripts; nor cited by many of the ancient fathers, even by such who wrote against the Arians, when it might have been of great service to them: to all which it may be replied, that as to the Syriac version, which is the most ancient, and of the greatest consequence, it is but a version, and a defective one. The history of the adulterous woman in the eighth of John, the second epistle of Peter, the second and third epistles of John, the epistle of Jude, and the book of the Revelations, were formerly wanting in it, till restored from Bishop Usher's copy by De Dieu and Dr. Pocock, and who also, from an eastern copy, has supplied this version with this text. As to the old Latin interpreter, it is certain it is to be seen in many Latin manuscripts of an early date, and stands in the Vulgate Latin edition of the London Polyglot Bible: and the Latin translation, which bears the name of Jerom, has it, and who, in an epistle of his to Eustochium, prefixed to his translation of these canonical epistles, complains of the omission of it by unfaithful interpreters. And as to its being wanting in some Greek manuscripts, as the Alexandrian, and others, it need only be said, that it is to be found in many others; it is in an old British copy, and in the Complutensian edition, the compilers of which made use of various copies; and out of sixteen ancient copies of Robert Stephens's, nine of them had it: and as to its not being cited by some of the ancient fathers, this can be no sufficient proof of the spuriousness of it, since it might be in the original copy, though not in the copies used by them, through the carelessness or unfaithfulness of transcribers; or it might be in their copies, and yet not cited by them, they having Scriptures enough without it, to defend the doctrine of the Trinity, and the divinity of Christ: and yet, after all, certain it is, that it is cited by many of them; by Fulgentius {z}, in the beginning of the "sixth" century, against the Arians, without any scruple or hesitation; and Jerom, as before observed, has it in his translation made in the latter end of the "fourth" century; and it is cited by Athanasius {a} about the year 350; and before him by Cyprian {b}, in the middle, of the "third" century, about the year 250; and is referred to by Tertullian {c} about, the year 200; 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; and besides, there never was any dispute about it till Erasmus left it out in the, first edition of his translation of the New Testament; and yet he himself, upon the credit of the old British copy before mentioned, put it into another edition of his translation"

So much for being added in the Middle Ages.

 

I love my good Calvinist brother John Gill, but I don't believe he was a expert on textual criticism. Bruce Metzger is well respected in this field.
　
http://www.bible-researcher.com/comma.html
　
We give below the comments of Dr. Bruce M. Metzger on 1 John 5:7-8, from his book, A Textual Commentary on the Greek New Testament, 2nd ed. (Stuttgart, 1993).
　
After μαρτυροῦντες the Textus Receptus adds the following: ἐν τῷ οὐρανῷ, ὁ Πατήρ, ὁ Λόγος, καὶ τὸ Ἅγιον Πνεῦμα· καὶ οὗτοι οἱ τρεῖς ἔν εἰσι. 8 καὶ τρεῖς εἰσιν οἱ μαρτυροῦντες ἐν τῇ γῇ. That these words are spurious and have no right to stand in the New Testament is certain in the light of the following considerations.


The passage is quoted by none of the Greek Fathers, who, had they known it, would most certainly have employed it in the Trinitarian controversies (Sabellian and Arian). Its first appearance in Greek is in a Greek version of the (Latin) Acts of the Lateran Council in 1215.

The earliest instance of the passage being quoted as a part of the actual text of the Epistle is in a fourth century Latin treatise entitled Liber Apologeticus (chap. 4), attributed either to the Spanish heretic Priscillian (died about 385) or to his follower Bishop Instantius. Apparently the gloss arose when the original passage was understood to symbolize the Trinity (through the mention of three witnesses: the Spirit, the water, and the blood), an interpretation that may have been written first as a marginal note that afterwards found its way into the text.

 

John Gill was an expert on ancient languages and commonly quoted from ancient translations, ancient writings (particularly ancient Jewish writings), etc. If he says it was referred to by someone around 200 A.D., I'll take his word for it. And yes, John Gill was an expert on the so called textual issues of the bible and dealt with many of them in his commentary.

As far as not having been quoted in response to heresy, consider this fact. When Jesus is getting on the Sadducees in Matthew 22 for not believing in the resurrection, He doesn't use any of the obvious resurrection texts in the OT, like Daniel 12:2 or Isaiah 26:19. Instead He quotes Exodus 3:6, which doesn't directly teach resurrection, but rather indirectly teaches it. Does that mean that Daniel 12:2 and Isaiah 26:19 aren't geniune, because (following your logic) Jesus would have most certainly used them to correct their denial of the resurrection? Of course not. Jesus is using a text these people have never considered relative to the subject of resurrection. No doubt the pharisees had been using Daniel 12:2 and Isaiah 26:19 and the sadducees had been swatting them down. Perhaps, just perhaps, those fighting against the heresy of denying the Triune Godhead were doing the same things.