John alone of the evangelists mentions the flowing of the blood and water from Christ's side; he alludes to it in his epistles, too. It is a beautiful testimony of divine grace, answering the last insult man could heap upon Him. They drove Him outside the camp, put Him to death on the cross, and then, to make assurance doubly sure, the soldier gives Him a blow with his spear. Salvation was God's answer to man's insult-sin in his rejection of Him. The blood and water were the signs of it.
In John's epistle the water is named first because, looked at on God's side, water comes first; in the history it cannot: "Forthwith came there out blood and water" (John 19:34); in the epistle, "Not by water only, but by water and blood." 1 John 5:6. The point is that eternal life is not found in the first Adam, but in the second; the witnesses to this are the water, the blood, and the Spirit. You want purifying to have eternal life; you will get it nowhere but in death, and in that of Christ in grace. You want expiation, and the blood of Christ makes that; you want the Holy Spirit.
Christ is not only dead but glorified, and the Spirit is given, the witness that there is no life in the first Adam, but in the Son. His power is found in that which marks the total breach of the first man with God and of God with him, save in sovereign mercy.
In the epistle, John is showing that moral cleansing will not be enough. The Spirit is named first when God applies it. The Word is the instrument, but it is by death itself. You must have cleansing, but the cleansing is death. The water coming forth from the side is purity, and you can have purity by death only, and by His death alone.