“Whoever confesses that Jesus is the Son of God, God abides in him, and he in God. So we have come to know and believe the love that God has for us. God is love, and whoever abides in love abides in God, and God abides in him” (1 John 4:15-21).
“Where does love come from?” The answer to this question is, “it depends on what you mean by ‘love.’” If you’re familiar with the Greek the New Testament was primarily written in, then you know that there were different words for the different kinds of love we experience. There was philia love which described the love we have for close friends. There was eros love which expressed a love driven by our passions. There was storge love which was associated with the love we have for family. Finally, there was agape love, which was characterized by unconditionality and sacrifice.
The agape, self-sacrificial love that God identifies with is not primarily a feeling; it is a person.
Though all the loves may be applied to God (depending on the context), the primary word for love we’re meant to associate with Him is agape. Indeed, in 1 John 4:15-21, the author uses the term sixteen times (!) in relation to our Lord. Like no other place in Scripture, John identifies God as synonymous with this agape love (“God IS love,” he writes in both vs. 8 and 16). But the agape, self-sacrificial love that God identifies with is not primarily a feeling; it is a person. Specifically, John says the source of this love is manifested in the person and work of Jesus Christ. If you want to know what God’s love is like you can only find it by looking to Jesus (vs. 15). This is what John means when he connects our confession that Jesus is the Son of God (vs. 15) with coming to “know and believe the love that God has for us” (vs. 16). To abide in the love of God is another way of saying, “to believe Jesus is who he says he is.”
Who is Jesus? The second person of the Trinity who forgives you all your sins and declares you righteous in his sight. He is the One who lives perfectly for you, dies substitutionally for you, raises victoriously for you, intercedes compassionately for you, and prepares a place zealously for you. He does it all for you because he loves you! After all, John 3:16 doesn’t say, “For God gave his only begotten that whoever believes in him should not perish but have eternal life. therefore, He can love you.” No, it says, “For God so loved the world that he gave his only begotten son that whoever believes in him should not perish but have eternal life.” The point is, his love for you precedes and fuels all that he has done for you in the person and work of Christ.
Can you believe it?!
At first blush, upon hearing such remarkable declarations, one might think, “Of course, I can believe that! Simple enough.” In truth, it is far easier said than done to believe the Lord of heaven and earth really does love sinful old you. As Martin Luther writes in his commentary on this passage:
God is love. These are simple words, but they are words that require faith in the highest degree – faith against which everything that is not of the Spirit of God fights. Conscience, the devil, hell, the judgment of God, and everything resist, so that we may not believe that God is love but may believe that God is an Executioner and a Judge.
He is the One who lives perfectly for you, dies substitutionally for you, raises victoriously for you, intercedes compassionately for you, and prepares a place zealously for you. He does it all for you because he loves you!
To believe God is love and thus loves you is a miracle wrought by the Holy Spirit. Yes, it’s a miracle and like all miracles, that means it is powerful. John continues:
By this is love perfected (literally completed or finished) with us so that we may have confidence for the day of judgment because as he is, so also are we in this world. There is no fear in love, but perfect love casts out fear. Fear has to do with punishment, and whoever fears has not been perfected in love (1 John 4:17-18).
How powerful is it to believe God loves you? It is powerful enough to take the greatest sinner who’s ever lived and fill them with fearless confidence to stand before the all-seeing, all-knowing God in judgment. Why? Because the beloved knows that God does not judge them from what he sees of their performance but solely on what he has seen from Christ’s performance.  As John writes, “Perfect love (Christ) casts out all fear.” To the degree that one can accept they are so loved, the weed of fear is cut out, and Spirit-empowered, fruitful love for the neighbor grows. Therefore:
We love because he first loved us. If anyone says, “I love God,” and hates his brother, he is a liar; for he who does not love his brother whom he has seen cannot love God whom he has not seen. And this commandment we have from him: whoever loves God must also love his brother. (1 John 4:19-21).
John’s point here is that since we are free (on account of Christ) from worry and fear over our standing with God, we can now look around and find ways to love and serve our neighbor. In fact, John says it’s impossible for one to say they “love God” and “hate their brother.” This all makes perfect sense. If we recognize how much God has loved sinners like us (despite our many shortcomings), then the result is that we’ll begin to extend such agape love to those around us. 
It sort of works like this:
A while back, I attended a very nice banquet held in honor of my church network’s President, Reverend Joel Egge. After many nice tributes and such, President Egge got up and talked about all the people he was grateful to for bringing him to where he was in life. At one point, he had his wife Barbara get up next to him, and he shared a simple but profound story. He said it was the night before his wedding to Barbara some 50-plus years ago. They were sitting next to each other, and he said something he probably shouldn’t have said to his fiancée. He said, “I’m just not really sure that you love me.” Shocked, she said, “Well, Joel, we’re getting married tomorrow. I should hope you know I love you.” Realizing he was in the doghouse, he said, “Barb, what I mean is, as you get to know more and more of the real me, as you see more and more of the mess that I am, it’s hard for me to believe that anybody could really love what they see.” Barbara said, “Well, Joel, I guess you’re just going to have to get used to the idea that I love you just because I love you.” He said, “She was such a picture, and always has been, of God’s love toward me.”
Yes, it was believing that he was indeed “loved just because he was loved” that propelled him to a life of love and service to the neighbor. The same is true for you. Never forget, “God does not love because of our works; He loves because of His love.”  And why is that? Because he is love.
 John writes in vs. 17 “as he is so also are we in this world”. What is Christ? Righteous. What are you through abiding (staying) in Christ? Righteous.
 See Matt. 18:21-35, “The Parable of the Unforgiving Servant”
 Luther, Martin, “Luther’s Works: The Catholic Epistles”, (Vol. 30, pg. 301)