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Albert Barnes and Edward D. Andrews write,
1 John 5:10 Updated American Standard Version (UASV)
10 The one who believes in the Son of God has the testimony in himself; the one who does not believe God has made him a liar, because he has not believed in the testimony that God has testified concerning his Son.
The one who believes in the Son of God has the testimony in himself. The evidence that Jesus is the Son of God. Comp. Notes, Rom. 8:16. This cannot refer to any distinct and immediate revelation of that fact, that Jesus is the Christ, to the soul of the individual, and is not to be understood as independent of the external evidence of that truth, or as superseding the necessity of that evidence; but the ‘witness’ here referred to is the fruit of all the evidence, external and internal, on the heart, producing this result; that is, there is 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 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, and make the sincere Christian ready to sacrifice anything rather than his religion; ready to go to the stake rather than to renounce his Savior. Comp. 1 Pet. 3:15.
The one who does not believe God has made him a liar. Because he has everywhere affirmed the depravity of all the race. Compare Rom. 1; 2; 3. On no point have his declarations been more positive and uniform than on the fact of the universal sinfulness of man. Compare Gen. 6:11-12; Job 14:4; Job 15:16; Psa 14:1-3; Psa 51:5; Psa 8:3; Rom. 3:9-20; Gal. 3:21.
Because he has not believed in the testimony that God has testified concerning his Son. 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, &c.—God had become a witness that the Lord Jesus was sent by him as a Savior, and that to doubt or deny this partook of the same character as doubting or denying any other testimony; that is, it was practically charging him who bore the testimony with falsehood.
 Albert Barnes, Notes on the New Testament: James to Jude, ed. Robert Frew (London: Blackie & Son, 1884–1885), 345–346.