His personal nature—“God is love” (1 John 4:8).
This statement does not say merely that love is of God, but that God is in His essential nature love (again the construction is anarthrous). Further, the statement does not imply that God’s being love is occasioned by anything. In other words, He is love apart from any opportunity to express it.
This love is the original love (1 John 4:10) because the source of it is God (1 John 4:19). It seems to be best illustrated by the earthly picture of the love within a family (1 John 4:7; also see Ephesians 3:15; 5:25).
Expression of love within the family is the resultant idea from “God is love.” John’s reasoning was very simple: God is love; therefore what God begets loves; thus Christians should show that they are begotten of God by loving one another.
This idea seems to have captivated John as he grew older. To him it was the central feature of Christianity. In no other book of the New Testament does the word love appear as often as it does in 1 John 3:1–5:12.
In the heart of that section, 4:11–21, John listed the important practical ramifications of our loving one another because God is love.
Love fulfills our duty (4:11); love can be realized in its most complete form when we love others (4:12b); love causes us to know the indwelling of the Holy Spirit (4:13–15); love gives us boldness in the day of judgment (4:17); love casts out fear (4:18); and love proves our profession of Christianity (4:19–21). It is easy to say, “I love God,” but it is often much more difficult to prove it by loving the brethren. This, however, is the expected result of the fact that we know God, who is love.
Charles C. Ryrie, Biblical Theology of the New Testament (Dubuque, IA: ECS Ministries, 2005), 291–292.