No man hath seen God at any time - See the notes at John 1:18, where the same declaration occurs. The statement seems to be made here in order to introduce a remark to show in what way we may know that we have any true knowledge of God. The idea is, “He has never indeed been seen by mortal eyes. We are not, then, to expect to become acquainted with what he is in that way. But there is a method by which we may be assured that we have a true knowledge of him, and that is, by evidence that we love another, and by the presence of his Spirit in our hearts. We cannot become acquainted with him by sight, but we may by love.”
If we love one another, God dwelleth in us - Though we cannot see him, yet there is a way by which we may be assured that he is near us, and that he even dwells in us. That way is by the exercise of love. Compare the notes at John 14:23-24.
And his love is perfected in us - Is carried out to completion. That is, our love for each other is the proper exponent of love to him reigning in our hearts. The idea here is not that we are absolutely perfect, or even that our love is perfect, whatever may be true on those points, but that this love to others is the proper carrying out of our love toward him; that is, without this our love to him would not have accomplished what it was adapted and designed to do. Unless it produced this effect, it would be defective or incomplete. Compare 1 John 4:17. The general sense is this: “We claim to have the love of God in our hearts, or that we are influenced and controlled by love. But however high and exalted that may seem to be as exercised toward God, it would be defective; it would not exert a fair influence over us, unless it led us to love our Christian brethren. It would be like the love which we might profess to have for a father, if it did not lead us to love our brothers and sisters. True love will diffuse itself over all who come within its range, and will thus become complete and entire.” This passage, therefore, cannot be adduced to demonstrate the doctrine of sinless perfection, or to prove that Christians are ever absolutely perfect in this life. It proves only that love to God is not complete, or fully developed, unless it leads those who profess to have it to love each other. See the notes at Job 1:1. On the meaning of the Greek word here used, (τελειόω teleioō,) see the notes at Philippians 3:12. Compare the notes at Hebrews 2:10.