Albert Barnes on 1 John 4:9

In this was manifested the love of God. That is, in an eminent manner, or this was a most signal proof of it. The apostle does not mean to say that it has been manifested in no other way, but that this was so prominent an instance of his love, that all the other manifestations of it seemed absorbed and lost in this.

Because that God sent his only begotten Son, etc. Jn 3:16.

That we might live through him. He died that we might have eternal life through the merits of his sacrifice. The measure of that love, then, which was manifested in the gift of a Saviour, is to be found,

(1.) in the worth of the soul;

(2.) in its exposure to eternal death;

(3.) in the greatness of the gift;

(4.) in the greatness of his sorrows for us; and,

(5.) in the immortal blessedness and joy to which he will raise us. Who can estimate all this? All these things will magnify themselves as we draw near to eternity; and in that eternity to which we go, whether saved or lost, we shall have an ever-expanding view of the wonderful love of God.

(b) "In this" Jn 3:16 (c) "that we might" Jn 6:51