Why does the apostle John witness to the water and the blood?
The Scriptures in question are as follows: “but coming to Jesus, when they saw that he was already dead they did not break his legs, but one of the soldiers pierced his side with a spear, and immediately there came out blood and water. And he who saw it bears witness and his witness is true, and he knows that he says true that ye also may believe” (John 19: 33-35). Again, “This is he that came by water and blood, Jesus [the] Christ; not by water only, but by water and blood. And it is the Spirit that bears witness, for the Spirit is the truth. For they that bear witness are three: the Spirit, and the water, and the blood; and the three agree in one” (1 John 5: 6-8). We should not separate these two Scriptures from one another: if we would understand 1 John 5: 6-8, then we must connect it to John 19: 33-35 and the death of Christ. John witnessed to the blood and water at Christ’s death and not at any other incident in his life. In John’s epistle, the Spirit is now the witness, but there is no reason to believe that there is any change in the incident which is referred to—the death of Christ.
Now the “Word became flesh” (John 1: 14)—that is, the One who was “in the beginning with God” (v2) entered in upon what He was not before. If He had not “come in flesh” (1 John 4: 2), then He could not have been “put to death in flesh” (1 Pet. 3: 18). Similarly, that same blessed One “came by water and blood” (1 John 5: 6), in order that those constituent elements of His body might be shed.
Blood and water both refer to cleansing. Thus the “blood of Jesus Christ his Son cleanses us from all sin” (1 John 1: 7). This is a judicial cleansing in that it deals with the matter of guilt and the demands of divine righteousness. Water refers to moral cleansing, and the Lord had illustrated the importance of this in John 13: “He that is washed all over needs not to wash save his feet, but is wholly clean; and ye are clean, but not all. For he knew him that delivered him up: on account of this he said, Ye are not all clean” (vs. 10, 11). Moral cleansing refers to the new nature imparted to the believer and the desires and interests that flow from that nature as acted upon by the Holy Spirit. Thus, after detailing the sins that mark man after the flesh, Paul can say of the Corinthian saints “And these things were some of you; but ye have been washed, but ye have been sanctified, but ye have been justified in the name of the Lord Jesus, and by the Spirit of our God” (1 Cor. 6: 11, my emphasis).
How are we cleansed with water? Some would refer us to baptism with water and Acts 22: 16: “Arise and get baptised, and have thy sins washed away, calling on his name”. Certainly, this is a cleansing, but it is a cleansing in the eyes of men. In this Scripture Saul of Tarsus is to sever himself from his previous sinful way of life by entering the waters of baptism, but the rite, in itself, produces no moral transformation. At best it is the outward sign of an inward change. The moral cleansing by water is by the Word of God. The Lord Himself tells us this in John 15: “Ye are already clean by reason of the word which I have spoken to you” (v3). This action of the Word commences in new birth, for we are “born again, not of corruptible seed, but of incorruptible, by [the] living and abiding word of God” (1 Pet. 1: 23; see James 1: 18). Thus, we are born, not only of Spirit, but “of water and of Spirit” (John 3: 5), for it is the Holy Spirit acting by means of the Word (here under the figure of water) that brings about new birth. There is such a thing as “the washing of water by [the] word” (Eph. 5: 26) which applies both when we are “washed all over” (John 13: 10), but also throughout our lives in relation to any defilement we may pick up while passing through this evil world (hence the Lord’s teaching on feet-washing in vs. 4-17).
Why does John say, “not by water only, but by water and blood” (1 John 5: 6)? The apostle wrote at a time when many were being baptised who did not really know the Lord. To conform, these would undergo a degree of outward moral change, while remaining the same as before on the inside. Some would eventually abandon all pretence of Christianity as in, “[The] dog [has] turned back to his own vomit; and, [The] washed sow to [her] rolling in mud” (2 Pet. 2: 22, my emphasis). By saying “not by water only” (1 John 5: 6) the apostle is addressing those who thought an outward moral reformation (or even just baptism) was enough. Hence, he goes on to say “but by water and blood”—that is, there needs to be that true moral cleansing that is inextricably bound up with the judicial cleansing only found in Christ.
To sum up: just as there could be no judicial cleansing for man if Christ had not died, so there could have been no moral cleansing. In Christ, not only is the believer cleared of all the charges against him, but his moral and spiritual compass is completely changed.