What does 1 John 5:7 mean?

This short verse simply emphasizes John's claim that there is evidence proving Jesus is the Son of God. In verse 6, these were defined as the water, the blood, and the Spirit. These refer to God's voice at Jesus' baptism, His crucifixion and resurrection, and the witness of the Holy Spirit. In verse 8, John will note that these three agree. In verse 9, John points out that the testimony of God is greater than that of men, making his a powerful and convincing argument that Jesus is the Son of God.

Some have suggested that water and blood refer to the Father and Son to make the case that all three Persons of the Trinity are involved in the testimony. While possible, this is unlikely since John is providing testimony that Jesus is God's Son. He would not use Jesus as testimony since He would be testifying for Himself (John 5:31). This would explain why John uses the concepts of water and blood as the two others who testify that Jesus is God's Son.

The transition between this verse and verse 8 is sometimes the subject of controversy. The source material of the King James Version, the Textus Receptus, adds a reference to "the Father, the Word and the Holy Ghost." This phrase is not found in any of the earliest manuscripts, and wasn't even in the earliest produced copies of the Textus Receptus. A likely explanation is that the passage was harmonized for use in liturgy. Over time, this harmonization found its way into manuscript copies. Once included in the Latin Vulgate, it became widespread.