John Calvin Commentary 1 John 4:12

"No man hath beheld God at any time: if we love one another, God abideth in us, and his love is perfected in us:" — 1 John 4:12 (ASV)

No man has seen God. These same words are found in the first chapter of John’s Gospel (John 1); but John the Baptist did not have exactly the same thing in view there, for he meant only that God could not be known in any other way, except as He has revealed Himself in Christ. The Apostle here extends the same truth further: that we comprehend the power of God by faith and love, so that we know we are His children and that He dwells in us.

He speaks, however, first of love, when he says that God dwells in us, if we love one another; for perfected, or really proved to be, in us is then His love; as though he had said that God shows Himself as present when by His Spirit He forms our hearts to entertain brotherly love. For the same purpose, he repeats what he had already said: that we know by the Spirit whom He has given us that He dwells in us; for it is a confirmation of the previous sentence, because love is the effect or fruit of the Spirit.

The sum, then, of what is said is this: since love is from the Spirit of God, we cannot truly and with a sincere heart love the brethren unless the Spirit puts forth His power. In this way, He testifies that He dwells in us. God by His Spirit dwells in us; therefore, by love, we prove that we have God abiding in us. On the other hand, whoever boasts that he has God and does not love the brethren, his falsehood is proved by this one thing: he separates God from Himself.

When he says, and His love is perfected, the conjunction is to be taken as causative: for, or, because. And love here may be explained in two ways: either that which God shows to us, or that which He implants in us. That God has given His Spirit to us, or given us of His Spirit, means the same thing, for we know that the Spirit is given in a measure to each individual.