I have written these things to you who believe in the name of the Son of God, so that you may know that you have eternal life.
13 These things have I written unto you As there ought to be a daily progress in faith, so he says that he wrote to those who had already believed, so that they might believe more firmly and with greater certainty, and thus enjoy a fuller confidence as to eternal life.
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
These things have I written unto you,.... Which are contained in the epistle in general, and particularly what is written in the context, concerning the victory of the world, being ascribed to him who believes that Christ is the Son of God; and concerning the six witnesses of his sonship, and the record bore by God, that the gift of eternal life is in him...