Have Ye Not Read?
Question: What is the "witness" the believer has in himself of 1 Jn. 5:10?
Reply: The verse reads, "He that believeth on the Son of God bath the witness in him: he that believeth not God hath made him a liar; because he hath not believed in the witness that God hath borne concerning his Son."
The "witness" that the believer has in him is the testimony that God has given concerning His Son. The word "witness,'' translated from the Greek word marturia, means "witness, testimony" (see Robert Young, J,H. Thayer,- Arndt and Gingrich, W.E. Vine, etc.). The Spirit,-the water and the blood had borne witness to Jesus (v. 8);their testimony is harmonious. Now, looking at verse 9, John states in the latter part of that verse, "for the witness of God is this, that he hath borne witness concerning his Son." God has given His testimony, declaring Jesus to be His Son. At the baptism of Jesus, God spoke from heaven saying, "This is my beloved Son in whom 1 am well pleased" (Matt. 3:17). God again bore testimony to this fact when He spoke out of the cloud at the transfiguration of Jesus. He said, "This is my beloved Son, in whom I am well pleased; hear ye him" (Matt. 17:5).
The testimony of God produces faith (Rom. 10:17. When one believes that testimony, he has it as a witness in his heart.
Guardian of Truth XXVI: 6, p. 82