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. 9 In this was manifested the love of God toward us, because that God sent his only begotten Son into the world, that we might live through him. 10 Herein is love, not that we loved God, but that he loved us, and sent his Son to be the propitiation for our sins. 11 Beloved, if God so loved us, we ought also to love one another.
Here John returns to his discussion of love, especially loving one another. There are many throwbacks to chapter three (for instance verse 4:7 with 3:11, and 4:8 with 3:16). 3:24-4:6 left the concept of love briefly discussing righteousness, and how you can tell a believer from a nonbeliever by whether or not that person is righteous. But even this doesn’t depart from the topic of love entirely, for 3:23-24 clearly show the commandment to love another is the law being treated in the topic of righteous.
Here (paralleling the movement from 3:10-15 to 3:16-23) the movement is from talking about discerning believers from unbelievers to an imperative to act like believers. And this action is not abiding by strict laws, but it is loving one another, for this is also the basis of discerning a believer from a nonbeliever.
And the language here is so powerful, that it stirs our hearts to desire to do the very thing asked of us. Again, like 3:16, the basis of knowing what love is is the sending of Christ. So love is not seen as an emotion, but as action. If I love you, that does not mean I desire you, or that you make me happy. It is that I am willing to sacrifice myself for your happiness.
Love is also seen as the only road to God. Jesus said that if we wish to follow Him, we must take up our cross and follow. I do not think that merely means that we must suffer for the gospel (though suffering is part of the Christian life). Instead, I believe it means that we must sacrifice our selves if we are to approach God. We sacrifice our wants, our needs, and if we are privileged enough, our very lives for the gospel and the things of God. This is the path of the Christian. But it is important that this isn’t to be motivated by obligation, or fear of hell. It is to be obligated by love: by the love of God and the love of others. For God is love, and if we are incapable of love, then we are incapable of knowing God.