What the texts say. The short text reads: “there are three who bear witness, the Spirit, and the water, and the blood: and the three agree in one” (ASV). The long text inserts a heavenly trio, Father, Word and Holy Spirit, and an earthly one, Spirit, water and blood. The KJV, Douay-Rheims and Reina-Valera 1909 have the long text; the ASV and almost all modern translations have the short one.
The Greek evidence. The added words are found in only about ten Greek manuscripts, all late: the oldest with the words in the text is from the fourteenth century, and several others have them only in the margin, added by later hands. No Greek writer is known to quote them before a Greek translation of the acts of the Fourth Lateran Council of 1215. That the words were not part of the original Greek letter is the conclusion of textual scholars across the traditions (Very Strong).
Where they came from. The words arose in the Latin West. The first undisputed quotation is in a Latin work of about 380, usually attributed to Priscillian. St Cyprian in the third century wrote that “these three are one” is written of the Father, Son and Holy Spirit, but he may have been interpreting verse 8 rather than quoting a longer text (Debated). The earliest surviving Vulgate manuscripts, such as Codex Fuldensis of 546, lack the words; later Vulgate copies gained them, and the Clementine Vulgate of 1592 printed them.
Erasmus and the printed Greek. Erasmus left the words out of his Greek New Testament in 1516 and 1519 because his Greek manuscripts lacked them. He added them in 1522 after a Greek manuscript containing them was reported, while saying in his notes that he suspected it had been corrected from the Latin. The popular story that he had promised to include them if one manuscript were produced has been shown to have no basis in his writings. His stated reason was to give no one an occasion for slander. From his later editions the words passed into the Textus Receptus and the KJV.
What the Orthodox Church reads. The Patriarchal edition of 1904 is unusually candid. Its editors wrote that the passage is unattested in the Church’s texts, in the Fathers and teachers of the Eastern Church, in the ancient versions and in the older Slavonic manuscripts. Yet they printed it in small type, explaining: “It is retained upon the opinion of the Holy Synod.” The Orthodox reader therefore finds the words in the Patriarchal Text, visibly marked as of a different standing.
Catholic and Protestant practice. In 1897 the Roman Holy Office ruled that the passage’s authenticity could not safely be denied; in 1927 this was declared open to scholarly investigation. The New Vulgate (Nova Vulgata) omits it. Modern Catholic and Protestant translations, including the RSV Catholic Edition, ESV, NIV and NRSVUE, omit it or place it in a note; the NKJV keeps it with a note.
What it does and does not change. The Church confessed the Holy Trinity at Nicaea and Constantinople without this verse; the Eastern Fathers who defended the Trinity never cited it. The doctrine rests on the whole witness of Scripture, such as baptism in the one name of the Father, the Son and the Holy Spirit (Matthew 28:19), and on the Church’s faith. Removing the Comma removes a late gloss, not the Trinity.