First, especially in our culture where sentimental or romantic notions of love have sometimes masked the richness of the biblical treatments of the subject, we must constantly remind ourselves that the double-love command [Luke 10:27] is deeply constrained by the double object. So far as the greatest command is concerned, we are not simply to love, to love in the abstract, but to love God. Nor does this mean that we are to love any god or the god of our choosing, but the God and Father of our Lord Jesus Christ. To love this God means, among many other things, that we will be hungry to get to know him better; conversely, in learning his words and ways, his attributes and his glory, what he loves and what he hates, we will find that our understanding of what it means to love God, what it means to love enemies, what it means to love brothers and sisters in Christ, will all be progressively modified and enriched. Precisely because, as created, dependent, and redeemed creatures, we are called to love our Creator, our Sovereign, our Redeemer with heart and soul and strength and mind, we will be firmly led to think robustly about what he is like, how he views evil, what rights and responsibilities he gives to the state in a fallen world, his role both in making peace and in judgment, and, above all, his commitment to his own glory as God. That is what forces us to avoid mere sentimentality. The fact that we are called to love this God and not, say, Allah, Shiva, or Marxism constrains the way we think about everything, including love.
Second, we dare not forget that although in his teaching the two love commands hang together, the Lord Jesus himself makes a distinction between the first and the second commandment. The first is to love God with heart and soul and mind and strength; the second is to love our neighbor as ourselves. The latter is neither the equivalent of the first nor a replacement for the first; nor should it be confused with the first. This is not a matter of mere counting, of mere prioritization. It is a matter of the structure of reality. God alone is God; God alone is our Maker and Redeemer; to God alone we acknowledge our absolute dependence. And then this God insists that we must love other creatures who have been made in his image as we love ourselves.' To reverse or confuse the first and second commandments is to return to idolatry by another route: it is to love the created order more than the Creator himself, who is blessed forever.
D. A. Carson in Love in Hard Places, page 187