Matthew 5:7 Blessed are the merciful: for they shall obtain
mercy. (kjv)
When a Christian has received mercy from Christ, the 'Bread of Life', he starts to become thankful and loving towards God. Because he comes to appreciate God’s merciful love towards him, he cannot help but love God. His love is born out of his trust in God's love by the Holy Spirit. This love of God and his mercy then helps to motivate him to become more merciful towards others.
1 John 4:19 We love him*, because he first loved us. (kjv)
Some modern scholars remove the word 'him' due to their textual preferences. Personally my textual preference is to keep it in. I'm not going to argue over manuscript evidence in this post, however, I believe this is the beginning of a logical argument through verses 19-21. First he loves us so we then love him. It is natural to reciprocate love to those who love us. This is common sense and fits with verse 19. When a Christian sees the mercy God gives him, he becomes thankful and begins to love him. As we learn to love him, we start to
conform to his desires. Because we love his desires we begin to
love those who he loves. This theme is developed in verses 20 and 21.
1 John 4:20 “If any man say, I love God, and hateth his
brother, he is a liar...” (kjv)
Verse 20 prevents misinterpretation of verse 19. It prevents us professing that we fully love him without the necessary evidence of loving our brother. Verse 20 helps us to understand what loving God really means. The evidence of loving him is when we love our brothers (verse 20).
It is a bit like this: if a teacher were to say to a class of children, "Have you all washed your hands?" Then, in response, all of the children put up their hands and shout, "Yes!" But then the teacher says, "Why then do your hands still look dirty?"
The evidence of true hand washing is for your hands to look clean. To apply the example we can say, the evidence of loving God is to love your brother.
In verse 21, again the emphasis is first a focus on loving God and then the evidence of that is to have love for your brother. In verse 19 it makes less sense to me to describe love as a generic, non-specific love, otherwise the progression of loving God to loving our brothers is less apparent. I believe the fruit of God's love is that we love him, and the fruit of loving him is to love your brother.
It is important to remember that no christian has perfect love. We will be made perfect by the Holy Spirit. When a Christian has faith in Christ, a process begins which is called progressive sanctification. God works in them so that they start to become practically righteous. When a Christian's love fails they are forgiven through faith in Christ.
agapáō 'to love': actively doing the Lord's desire
The type of love we need to have is love which reflects the love Christ has for his people, namely a love that is self-sacrificial and forgiving, love that shows mercy.