John Calvin Commentary 1 John 4:9

"Herein was the love of God manifested in us, that God hath sent his only begotten Son into the world that we might live through him." — 1 John 4:9 (ASV)

In this was manifested, or, has appeared. The love of God towards us is also testified by many other proofs. For if it is asked why the world was created, why we have been placed in it to possess the dominion of the earth, why we are preserved in life to enjoy innumerable blessings, or why we are endowed with light and understanding, no other reason can be adduced except the gratuitous love of God.

But the Apostle here has chosen the principal evidence of it, and what far surpasses all other things. For it was not only an immeasurable love that God did not spare his own Son, so that by his death he might restore us to life; but it was a most marvelous goodness, which ought to fill our minds with the greatest wonder and amazement. Christ, then, is so illustrious and singular a proof of divine love towards us, that whenever we look upon him, he fully confirms to us the truth that God is love.

He calls him his only begotten to amplify. For in this he more clearly showed how singularly he loved us, because he exposed his only Son to death for our sakes. In the meantime, he, who is his only Son by nature, makes many sons by grace and adoption—namely, all who by faith are united to his body. He expresses the purpose for which Christ was sent by the Father: that we may live through him. For without him we are all dead, but by his coming he brought life to us; and unless our unbelief prevents the effect of his grace, we feel it in ourselves.