1 John 5:10

He that believeth on the Son of God
As a divine person who came in the flesh, and obeyed the law, and brought in everlasting righteousness, and obtained life and salvation for men: he that with the heart believes in him for righteousness, and eternal life, he being the Son of God, truly and properly God, and so able to save all that believe in him,

hath the witness in himself;
of the need he stands in of Christ, and of the suitableness, fulness, and excellency of him; the Spirit of God enlightening him into the impurity of his nature, his impotence to do anything spiritually good, his incapacity to atone for sin, and the insufficiency of his righteousness to justify him before God; and convincing him that nothing but the blood of the Son of God can cleanse him from sin, and only his sacrifice can expiate it, and his righteousness justify him from it, and that without him he can do nothing; testifying also to the efficacy of his blood, the completeness of his sacrifice and satisfaction, the excellency of his righteousness, and the energy of his grace and strength: so he comes to have such a witness in himself, that if ten thousand arguments were ever so artfully formed, in favour of the purity of human nature, the power of man's free will, and the sufficiency of his righteousness, and against the sacrifice and righteousness of Christ, the dignity of his person, as the Son of God, which gives virtue to his blood, sacrifice, and righteousness, they would all signify nothing to him, he would be proof against them. And such an one very readily receives into him the testimony God gives of his Son, of the glory and excellency of his person, and retains it in him. The Alexandrian copy and the Vulgate Latin version read, "hath the witness of God in him"; to which the Ethiopic, version agrees, and confirm the last observation:

he that believeth not God;
does not receive his testimony concerning his Son: the Alexandrian copy, and two of Stephens's, and the Vulgate Latin version read, "he that believeth not the Son"; and the Ethiopic version, his Son; and the Arabic version, "the Son of God"; and so is a direct antithesis to the phrase in the former clause of the verse:

hath made him a liar;
not the Son, but God, as the Arabic version renders it, "hath made God himself a liar"; who is the God, of truth, and cannot lie; it is impossible he should; and as nothing can be, more contumelious and reproachful to the being and nature of God, so nothing can more fully expose and aggravate the sin of unbelief, with respect to Christ, as the Son of God:

because he believeth not the record that God gave of his Son;
at the times and places before observed.