Albert Barnes Commentary 1 John 5:10

"He that believeth on the Son of God hath 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." — 1 John 5:10 (ASV)

He that believeth on the Son of God hath the witness in himself. The evidence that Jesus is the Son of God. (See the notes on Romans 8:16).

This cannot refer to any distinct and immediate revelation to the soul of the individual that Jesus is the Christ. Nor is it to be understood as independent of the external evidence of that truth, or as superseding the need for that evidence. Instead, the "witness" referred to here is the fruit of all the evidence, external and internal, on the heart, producing this result: namely, the deepest conviction of the truth that Jesus is the Son of God.

There is the evidence derived from the fact that the soul has found peace by believing on him; from the fact that the troubles and anxieties of the mind on account of sin have been removed by faith in Christ; from the new views of God and heaven which have resulted from faith in the Lord Jesus; from the effect of this faith in disarming death of its terrors; and from the whole influence of the gospel on the intellect and the affections—on the heart and the life. These things constitute a mass of evidence for the truth of the Christian religion, whose force the believer cannot resist, making the sincere Christian ready to sacrifice anything rather than his religion, ready to go to the stake rather than renounce his Saviour. (See the notes on 1 Peter 3:15).

He that believeth not God hath made him a liar. (See the notes on 1 John 1:10).

Because he believeth not the record, etc. The idea is that in various ways—at his baptism, at his death, by the influences of the Holy Spirit, by the miracles of Jesus, etc.—God had become a witness that the Lord Jesus was sent by him as a Saviour. To doubt or deny this shared the same character as doubting or denying any other testimony; that is, it was practically charging him who bore the testimony with falsehood.