What does 1 John 5:8 mean?

This verse adds the identities of the three who testify, and emphasizes their agreement. A textual controversy exists between the end of verse 7 and the beginning of verse 8. The Textus Receptus, which was used in translating the King James Version, adds a reference to a witness on earth. While used in the majority of later handwritten Greek manuscripts, this reading is not found in the earliest manuscripts. Nor was it in the earliest versions of the Textus Receptus, being added later in the publishing process.

It is likely that this difficult passage was harmonized in early liturgy, which are readings adapted for worship gatherings. This harmonization, the disputed portion between verses 7 and 8, was likely used later in manuscripts which became part of the majority text. The majority text was greatly popularized when included in the Latin Vulgate. Interestingly, the early German translations of the Bible did not include the reference to a witness on earth found in the Textus Receptus, so this particular controversy is most common among English speaking interpreters of Scripture.