1 John 4:7-8
7 Beloved, let us love one another: for love is of God; and every one that loveth is born of God, and knoweth God.
8 He that loveth not knoweth not God; for God is love.
The Grace New Testament Commentary - 1 John 4:7-8
4:7 The apostle now leaves behind his discussion about the many false spirits which "have gone out into the world" to allure Christians into worldly ideas. These ought to be rejected so that the readers can focus on love for one another.
If the readers obeyed the command to love one another, they would be carrying on an activity that was distinctively sourced in their heavenly Father. The reason for this is that love itself is of God.
It follows then that two things may be safely said of everyone who loves: (1) such a person is born of God and (2) he also knows God. John treats these concepts as two different things, for he says in v 8 that "he who does not love does not know God." It would have been easy to say, "He who does not love is not born of God and does not know God" in direct antithesis to the statement of v 7. But this is precisely what cannot be said. Already John has talked about a person who "hates his brother," which is a feat quite impossible for a non-Christian since a Christian is not his brother (cf. 1 John 2:11; 3:10; 3:15; 4:20). Those who teach that a Christian cannot hate another Christian are teaching a myth.
4:8 But since a born-again Christian can fail to love, if he does not love it would show that he has not really come to know his heavenly Father (cf. 1 John 2:3). The God who has begotten him is love. Here is the second of 1 John's two great affirmations about God. The first, in 1 John 1:5, affirms that "God is light." Now John declares that God is love. The former points to His perfect holiness; His freedom from all sin or deception. This second statement affirms that His basic nature is characterized by love. This does not mean God has no other attributes, such as wisdom and justice. But it does indicate that love is fundamental to what God is and to what He does.