Summary: John speaks of the three-fold witness of the Holy Spirit.
The Bible says the child of God has a "know-so" salvation. We can know beyond a shadow of a doubt that we are a child of God. John speaks about the witness of the Holy Spirit, who assures God's children they belong to Him. First of all, the Holy Spirit bears witness:
1. To The Son - vs. 6-7
"But when he, the Spirit of truth, comes, he will guide you into all the truth. He will not speak on his own; he will speak only what he hears, and he will tell you what is yet to come. He will glorify me because it is from me that he will receive what he will make known to you. All that belongs to the Father is mine. That is why I said the Spirit will receive from me what he will make known to you."
- John 16:13-15 (NIV)
A. The Holy Spirit bears witness to the Lord's coming for us.
The Spirit witnesses to the fact that Jesus is God come in human flesh to make Himself to known as we could never know Him, and to do for what us what we could never do for ourselves, pay the price for our sin.
John combated the Cerenthian heresy when he wrote this letter and his Gospel, which said Jesus was only a man. But at His baptism, the eternal Christ came upon Him, and was with Him until the crucifixion, where the Christ was withdrawn, so that Jesus died as a mere man. Jesus came by "water and blood," alluding to the beginning and end of His ministry. Jesus' ministry started with His baptism. The Spirit descended upon Him in the form of a dove, and the Father said, "This is my beloved Son, in whom I am well pleased" (Matthew 3:17). John says Jesus came by blood - a reference to the cross. The point is, Jesus was just as much the Christ when He died on the cross as when He was baptized. He never ceased to be the Christ, the eternal Son of God.
Jesus was God before His birth (John 1:1); Jesus was God at His birth (John 1:14); Jesus was God at His baptism; Jesus was God on the cross; Jesus was God in His resurrection; & Jesus is God for all eternity! God was intimately involved in the sacrifice of Calvary!
"Here is a trustworthy saying that deserves full acceptance: Christ Jesus came into the world to save sinners - of whom I am the worst."
- 1 Timothy 1:15 (NIV)
The Spirit testifies that the cross was no accident or "plan B"; it was the primary purpose of God coming in human form through Jesus!
B. The Holy Spirit bears witness to the Lord's commitment to us.
In John 19:34, says water and blood came from the wound in Jesus' side after His death. Why was this? Jesus was scourged prior to His crucifixion. The whip, or flagrum, consisted of braided leather strips with metal balls and pieces of bone woven into the braids. The balls caused deep bruising. The pieces of bone cut the flesh. The cuts exposed skeletal muscles, veins, sinews, and bowels of the victim. Beating were often so severe, victims wouldn't survive.
Those who did survive often went into hypovolemic shock, due to low blood volume. The heart would race to pump blood that wasn't there; the victim would collapse or faint due to low blood pressure; the kidneys would shut down to preserve body fluids; and they would experience extreme thirst as the body sought to replenish lost fluids. Scripture indicates Jesus experienced hypovolemic shock as a result of being flogged.