Whoever believes in the Son of God has the testimony in himself. Whoever does not believe God has made him a liar, because he has not believed in the testimony that God has borne concerning his Son.
9 For this is the witness, or testimony, of God The particle ὅτι does not mean here the cause, but is to be taken as explanatory; for the Apostle, after having reminded us that God deserves to be believed much more than men, now adds, that we can have no faith in God, except by believing in Christ, because God sets him alone before us...
In those words we may observe, I. The privilege and stability of the real Christian: He that believeth on the Son of God, hath been prevailed with unfeignedly to cleave to him for salvation, hath the witness in himself, Jo1 5:10. He hath not only the outward evidence that others have, but he hath in his own heart a testimony for Jesus Christ.
Commenting on 1 John 5:10-13
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...