But still, Jesus does say that treating others as you want to be treated “is the Law and the Prophets.” He does not say here that two commandments are the Law and the Prophets, but only one. This seems significantly different from Matthew 22:40 where the Law and the Prophets depend on both commandments.
Why does he say it in this way? I think what he means is that when you see people love like that (fulfill the Golden Rule), what you are seeing is the visible expression of the aim of the Law and the Prophets. This behavior among people manifests openly and publicly and practically what the Old Testament is about. It fulfills the Law and the Prophets by making the aim visible. Loving God, however, is invisible. It is an internal passion of the soul. But it comes to expression when you love others. So loving others is the outward manifestation, the visible expression, the practical demonstration, and therefore the fulfillment of loving God and therefore of what the Old Testament is about.
So there is a sense in which the second commandment (to love your neighbor) is the visible goal of the whole Word of God. It’s not as though loving God is not here, or that loving God is less important; rather, loving God is made visible and manifest and full in our visibly, practically, sacrificially loving others. I think that is why the second commandment stands by itself here as an expression of what the Law and the Prophets are—“… for this is the Law and the Prophets.” Loving our neighbor is not the Law and the Prophets independently of loving God. Rather, loving our neighbor is based on our love for God and, as the overflow of it, is what the Law and the Prophets were aiming at.
John Piper, What Jesus Demands from the World (Wheaton, IL: Crossway Books, 2006), 252–253.
I have just completed a series of lessons on the theme of Loving God. They are available on Amazon in both print and Kindle versions, as well as part of my Good Questions Have Groups Talking Subscription service. For a medium-sized church, lesson subscriptions are only $10 per teacher per year.