John writes about Jesus the Christ when he declares that He came both by water and by blood. The Spirit of God is the witness because He is the Spirit of truth. There are three witnesses in heaven and also on earth followed by the testimony.
Witness in Heaven
John says there are three witnesses in heaven. These are the Father, the Word and the Spirit. They are three but they are also one.
Witness on Earth
In 1 John 5: 8, he writes that there are also three witnesses on earth. The Spirit, the water and the blood. They are three but they all agree as if they were one. When the soldier struck Jesus to see if He was dead, water and blood spilled from His side. We may receive the witness of men but how much greater the witness of God who at His baptism stated, “This is my beloved Son in whom I am well pleased.”
Do you believe in the Son of God? That in itself is a witness. Otherwise you make God out to be a liar. The testimony is that God has given eternal life, the life that is in His Son. Anyone who has the Son also has His life. There are three witnesses in heaven and also on earth followed by the testimony.
John clarified the reason he wrote. He was writing to believers. He wanted them to know that they had eternal life and to give them a reason to continue to believe in the name of the Son of God.