It is commonly known that Erasmus did not include a large section of 1 John 5:7 in the 1st and 2nd Editions of his Greek New Testament.  This is the so called Comma Johanneum,  “For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one.” (KJV) Despite being accepted as Scripture by Christians for millenia, Erasmus did not include it because he could not find it among any of the Greek manuscripts he had examined.  A popular view states that Erasmus made a wager that if a manuscript was discovered he would include it.  The story goes that a manuscript was quickly produced and given to Erasmus who reluctantly included it in his 3rd and subsequent editions.
The story is actually a complete fabrication.  There is zero evidence that Erasmus ever made this wager and Erasmian scholar Dr. H.J. De Jonge refuted it way back in 1980.  Check out Chris Thomas’ post here for more information.  Even though the wager theory has been refuted, it continues to persist.  It is not only repeated in popular circles, but scholarly ones as well.  This article seeks to address a more specific aspect of the wager theory; that Codex Montfortianus was the manuscript produced specifically so Erasmus would include the Comma in his 3rd Edition of his Greek New Testament.  At the outset, I wish to thank Robert Paul Wieland for much of the information in this article and for his assistance with the Latin translations.
 
BACKGROUND
Before we get into the specifics, let us clarify some things regarding Erasmus and his 3rd Edition.  First, Erasmus did not just produce a Greek New Testament.  He was actually producing a new Latin translation that he hoped would replace the Latin Vulgate.  In order to justify his new translation, he put on the opposite page the Greek text he was translating from.  The Renaissance had brought with it a greater interest in original  documents.  The phrase “ad fontes” (Latin for “to the sources”) became a popular cry.  Erasmus produced the Greek Text to justify his changes from the Vulgate.  What is very significant is that he also included his translation notes in the back of the book as well.  These “Annotations” included information on which manuscripts he acquired certain readings from.  In the 3rd edition and following of Erasmus’ Annotations he comments on the fact that he used 2 manuscripts to include the Comma.  He called them the Hispanic Codex and the British Codex.
The Hispanic Codex is believed to be the Complutensian Polyglot.  This was another Greek New Testament that was being produced at the time.  Like Erasmus’ work, it was also a collation of many manuscripts available during that time.  It was produced by Catholic scholars in Spain, which is why most scholars have believed that was the Hispanic Codex Erasmus used.  The British Codex has traditionally been identified as Codex Montfortianus.  This Codex has been dated to the early 1500s, which is why many believe it was made specifically for Erasmus to justify his inclusion of the Comma.  Some believe that it was copied from a manuscript that did not include the Comma, which they claim was added from the Latin.
 
THE PROBLEM
While at first glance many of these views seems reasonable, but there is a very serious problem with identifying Erasmus’ British Codex with Codex Montfortianus.  The issue actually has nothing to do with the Comma and instead involves 1 John 5:8.

The Complutensian Polyglot Volume 4, page 413.

In Erasmus’ 4th and 5th editions of his Annotations, he adds some comments on a few of the differences between the manuscripts he used for 1 John 5:7-8.  Regarding verse 8, he says these words, “postremo quod Britanicus etiam in terrae testimonio adijciebat καὶ οἱ τρεῖς εἰς τὸ ἕν εἰσὶ”, which translated into English says, “Finally, the British even adds the witness on earth,

[and these three agree in one.]” (brackets are the KJV rendering of the Greek phrase).  Thus, we find that according to Erasmus.  The Hispanic Codex was missing the last phrase of 1 John 5:8, but the British Codex did include it.
When we look at the Complutensian Polyglot, we find that it agrees with Erasmus’ observation.  The last phrase of verse 8 is missing.  You can verify for yourself in Volume 4 of the Complutensian Polyglot here (page 413 of the pdf).
For comparison, Stephanus 1550 renders vs 7-9 as such:
” 7 ὅτι τρεῖς εἰσιν οἱ μαρτυροῦντες εν τῷ οὐρανῷ, ὁ πατήρ, ὁ λόγος, καὶ τὸ Ἅγιον Πνεῦμα· καὶ οὗτοι οἱ τρεῖς ἕν εἰσιν 8 καὶ τρεῖς εἰσιν οἱ μαρτυροῦντες ἕν τῇ γῇ, τὸ πνεῦμα καὶ τὸ ὕδωρ καὶ τὸ αἷμα καὶ οἱ τρεῖς εἰς τὸ ἕν εἰσὶν 9 εἰ τὴν μαρτυρίαν τῶν ἀνθρώπων λαμβάνομεν ἡ μαρτυρία τοῦ θεοῦ μείζων ἐστίν· ὅτι αὕτη ἐστὶν ἡ μαρτυρία τοῦ θεοῦ ἥν μεμαρτύρηκεν περὶ τοῦ υἱοῦ αὐτοῦ”
So far so good, right?
Now if the story is true that Codex Montfortianus is the British Codex, we should find the phrase καὶ οἱ τρεῖς εἰς τὸ ἕν εἰσὶ (and these three agree in one) at the end of 1 John 5:8.  If not, it means that the British Codex was not a 16th century manuscript made to order just to justify Erasmus including the Comma into his Greek New Testament.
Well, let’s take a look at Codex Montfortianus and see what it says…

The last phrase of verse 8 seems to be missing from Codex Montfortianus as well.  You can verify yourself by going here and looking at Folio 439 of Codex Montfortianus.
Since Codex Montfortianus is missing the last phrase of verse 8, that means Erasmus did not use it to justify the Comma.
Now, that begs the question. If it wasn’t Codex Montfortianus, which manuscript did Erasmus use?  I don’t have an answer to that question.  Suffice to say, we don’t know what happened to the British Codex.  It could have been destroyed in a fire or it could have been lost and is sitting in a box in someone’s attic.  Who knows!
The point of this article is not to identify the British Codex, but to refute the common myth that Erasmus used a recently produced manuscript which many claim was copied from a manuscript that did not have the comma, but was added by the scribe who produced Codex Montfortianus.
All these arguments are just attempts to undermine Erasmus’ work in an attempt to undermine the Received Text of the New Testament.  As has been shown in this case, those efforts have failed miserably.
To further add to the puzzlement of how this view became so widespread.  The inconsistency between Erasmus’ Annotations and Codex Montfortianus was mentioned at least 200 years ago by William Evanson in his Translator’s Preface to his translation of Francis Knittel’s New Criticisms on the Celebrated Text, 1 John V.7.