Today we get to one of the, if not the, most debated passages in 1 John- 5:6-8- “6This is the one who came by water and blood—Jesus Christ. He did not come by water only, but by water and blood. And it is the Spirit who testifies, because the Spirit is the truth. 7 For there are three that testify: 8 the Spirit, the water and the blood; and the three are in agreement.” What does water and blood mean?
One of the reasons for John’s writing of his letter was to refute Doecetism- the belief that Jesus did not come in the flesh, but was spirit in human form. John uses the concepts of Jesus coming by “water and blood” to affirm he was both flesh and divine. The exact meaning of the phrase has been debated. Some believe the water indicates his divine side, and the blood his coming in the flesh. Some believe the water indicates his physical birth and blood refers to the blood that was shed when he died. Others hold it is a reference to his baptism (water) and his death (blood). Still others believe it is only a reference to his death (blood and water flowing out of his side when pierced).
Whatever the meaning John is using the phrase to indicate that Jesus was both God and human. And the water and the blood, as well as the Holy Spirit testify to that fact.
For a more comprehensive read on the subject, this is a good link: http://www.academia.edu/749523/Jesus_came_through_water_and_blood