The Spirit, the Water, and the Blood

1 John
The Spirit, the water, and the blood agree. John used these three to testify to Christ because the Old Testament law stipulated that there had to be two or three witnesses to a matter in order to determine the truth.  Interestingly, later translations of these verses - 1 John 5:7-8 - have added a few lines about how the Father, the Word, and the Holy Spirit testify in heaven and the Spirit, water, and blood testify on earth.  However, this addition does not appear in any manuscript prior to the fourteenth century.  There's an interesting article by Daniel Wallace on bible.org (click here) that talks about this issue at greater length.  The reason this later addition has been kept is because it speaks to the Trinity, and yet it isn't actually very reliable.  However, I'm not an expert on these matters, so if textual criticism is your thing, check out the link above.