1 John 5:6
 This is he who came by water and blood—Jesus Christ; not by the water only but by the water and the blood. And the Spirit is the one who testifies, because the Spirit is the truth. (ESV)
The Apostle John is one of the greatest defenders of the deity of Jesus Christ. This important doctrinal truth was already under attack in the first century church. Should that be surprising to us? Of course not. Yesterday we looked at the great enemy of our faith, the deceiver. He has been a liar from the beginning. He is always at work trying to deceive people and distort the truth.
Dr. John MacArthur gives us a little insight into the actual form of heresy that John was addressing here in our verse today:
"According to early tradition (Irenaeus), another form of this heresy that John may have attacked was led by a man named Cerinthus, who contended that the Christ’s “spirit” descended on the human Jesus at his baptism but left him just before his crucifixion. John wrote that the Jesus who was baptized at the beginning of his ministry was the same person who was crucified on the cross".
- Dr. John MacArthur, Bible Study Notes
Jesus came by water and blood. John is referring to Christ's baptism (water) and crucifixion (blood). These two events both demonstrate the purpose of Christ's coming. Jesus came to save sinners. Water baptism is an ordinance given to us that demonstrates washing away of sin. The Lord's Supper is an ordinance that commemorates the sacrifice for our sins. Both of these ordinances practiced by churches all over the world testify to Jesus Christ who came by water and blood.
The next time you witness a water baptism or celebrate the Lord's Supper celebrate the reality of Jesus Christ, God became flesh, the water and the blood testify to this truth.