The NET footnote says "Other ancient authorities read (with variations) There are three that testify in heaven, the Father, the Word, and the Holy Spirit, and these three are one. And there are three that testify on earth:" Where do you get the idea that it was a marginal note that was added later? My commentaries state that it is found in only a few of the early manuscripts, but that doesn't invalidate it.
"However, it is highly unlikely that the
Comma Johanneum was originally a part of 1 John. None of the oldest Greek manuscripts of 1 John contain the comma, and none of the very early church fathers include it when quoting or referencing
1 John 5:7-8. The presence of the
Comma Johanneum in Greek manuscripts is actually quite rare until the 15th century A.D. It is primarily found in Latin manuscripts. While some of the Latin manuscripts containing the
Comma Johanneum are ancient, the
Comma Johanneum did
not appear in the original Latin Vulgate written by Jerome.
In the 16th century, when Desiderius Erasmus was compiling what became known as the Textus Receptus, he did not include the
Comma Johanneum in the 1st or 2nd editions. Due to intense pressure from the Catholic Church and others who wanted it included because of its support for trinitarianism, Erasmus included the
Comma Johanneum in later editions of the Textus Receptus. His decision resulted in the
Comma Johanneum being included in the King James Version of the Bible and later in the New King James Version. None of the modern Greek texts (UBS 4,
Nestle-Aland 27, Majority Text) contain the
Comma Johanneum. Of all the modern English translations, only the New King James Version and Modern English Version include the
Comma Johanneum."
https://www.gotquestions.org/Comma-Johanneum.html
Adam Clarke's Commentary states, in part:
'But it is likely this verse is not genuine. It is wanting in every MS. of this epistle written before the invention of printing, one excepted, the Codex Montfortii, in Trinity College, Dublin: the others which omit this verse amount to one hundred and twelve.
It is wanting in both the Syriac, all the Arabic, Ethiopic, the Coptic, Sahidic, Armenian, Slavonian, etc., in a word, in all the ancient versions but the Vulgate; and even of this version many of the most ancient and correct MSS. have it not. It is wanting also in all the ancient Greek fathers; and in most even of the Latin.
The words, as they exist in all the Greek MSS. with the exception of the Codex Montfortii, are the following: -
“1Jn_5:6. This is he that came by water and blood, Jesus Christ; not by water only, but by water and blood. And it is the Spirit that beareth witness because the Spirit is truth.
1Jn_5:7. For there are three that bear witness, the Spirit, the water, and the blood; and these three agree in one.
1Jn_5:9. If we receive the witness of man, the witness of God is greater, etc.”'
The Jamieson, Fausset, Brown Commentary:
"Two or three witnesses were required by law to constitute adequate testimony. The only Greek manuscripts
in any form which support the words, “in heaven, the Father, the Word, and the Holy Ghost, and these three are one; and there are three that bear witness in earth,” are the
Montfortianus of Dublin, copied evidently from the
modern Latin Vulgate; the
Ravianus, copied from the
Complutensian Polyglot; a manuscript at Naples, with the words added in the
Margin by a recent hand;
Ottobonianus, 298, of the fifteenth century, the
Greek of which is a mere translation of the accompanying
Latin. All the old versions omit the words. The oldest manuscripts of the
Vulgate omit them: the earliest
Vulgate manuscript which has them being
Wizanburgensis, 99, of the eighth century. . . . It was therefore first written as a
marginal comment to complete the sense of the text, and then, as early at least as the eighth century, was introduced into the text of the
Latin Vulgate."