That ye may know that ye have eternal life - I write to show your privileges - to lead you into this holy of holies - to show what believing on the Son of God is, by the glorious effects it produces: it is not a blind reliance for, but an actual enjoyment of, salvation; Christ living, working, and reigning in the heart.
And that ye may believe - That is, continue to believe: for Christ dwells in the heart only by Faith, and faith lives only by Love, and love continues only by Obedience; he who Believes loves, and he who Loves obeys. He who obeys loves; he who loves believes; he who believes has the witness in himself: he who has this witness has Christ in his heart, the hope of glory; and he who believes, loves, and obeys, has Christ in his heart, and is a man of prayer.
These things have I written unto you - The things in this Epistle respecting the testimony borne to the Lord Jesus.
That believe on the name of the Son of God - To believe on his name, is to believe on himself - the word “name” often being used to denote the person. See the notes at Matthew 28:19.
That ye may know that ye have eternal life - That you may see the evidence that eternal life has been provided, and that you may be able, by self-examination, to determine whether you possess it. Compare the notes at John 20:31.
And that ye may believe - That you may continue to believe, or may persevere in believing. He was assured that they actually did believe on him then; but he was desirous of so setting before them the nature of religion, that they would continue to exercise faith in him. It is often one of the most important duties of ministers of the gospel, to present to real Christians such views of the nature, the claims, the evidences, and the hopes of religion, as shall be adapted to secure their perseverance in the faith. In the human heart, even when converted, there is such a proneness to unbelief; the religious affections so easily become cold; there are so many cares pertaining to the world that are suited to distract the mind; there are so many allurements of sin to draw the affections away from the Saviour; that there is need of being constantly reminded of the nature of religion, in order that the heart may not be wholly estranged from the Saviour. No small part of preaching, therefore, must consist of the re-statement of arguments with which the mind has been before fully convinced; of motives whose force has been once felt and acknowledged; and of the grounds of hope and peace and joy which have already, on former occasions, diffused comfort through the soul. It is not less important to keep the soul, than it is to “convert” it; to save it from coldness, and deadness, and formality, than it was to impart to it the elements of spiritual life at first. It may be as important to trim a vine, if one would have grapes, as it is to set it out; to keep a garden from being overrun with weeds in the summer, as it was to plant it in the spring.