This is considered one of the more difficult passages of the New Testament to exegete.
"For there are three that bear record in heaven, the Father, the Word, and the Holy Ghost: and these three are one. And there are three that bear witness in earth, the Spirit, and the water, and the blood: and these three agree in one."
"It is probably best to see these as references to Jesus' power to make alive (Spirit), cleanse the believer from sin (water) and atone (blood). There is support for this symbolism in the Gospel of John (6:63, 13:10) and in 1 John (1:7, 2:2). It is also supported from the Old Testament prophecy of Zech 12:10–13:1, part of which is quoted in John 19:37. But how does all this relate to the author's debate with the opponents? If (as we have suggested previously) the debate centered over the saving significance of what Jesus did during his earthly life and ministry (especially his death on the cross), then the continuing presence of Jesus' power in the church to which the author is writing is experienced by believers as they are made alive (by the Spirit), cleansed from their sins (represented by water) and reconciled to God (by Jesus' death on the cross). These three things are witnesses because although the opponents can deny the apostolic eyewitness testimony regarding the importance of Jesus' earthly life and ministry they cannot deny the present effects of Jesus' actions in the lives of believers within the congregation." -W. Hall Harris III 
- Does 1 John 5:7 Teach The Trinity?, by Don Stewart
- The Textual Problem in 1 John 5:7-8, by Daniel B. Wallace
- The Comma Johanneum and Cyprian, by Daniel B. Wallace
- Is it true that 1 John 5:7 is not in any Greek manuscript before the 1600s?, by David W. Daniels
- Exegetical Commentary on 1 John 5:4b-12, by W. Hall Harris III