The Pharisees, seeing Jesus humble the Sadducees, think they can corner Him. In asking which commandment of the law is greatest they are trying to trap Jesus into claiming that one part of God’s Word is greater than another: all the commandments are great, because they come from God. You must follow the all the law, or none of it will avail you.
But Jesus’ response not only shocks them, but demonstrates His mastery of the law and, to us who believe in Him, His role as the giver of that law in ancient days, being He is the Word by Whom God gave the law. Quoting the ancient shema from Deuteronomy 6:5, Jesus sums up the first three commandments—worshiping God alone, not using God’s name in vain, and keeping the sabbath—and then Leviticus 18:19 which summarizes the rest of the commandments pertaining to our relationships with other people. By pointing to two verses of Scripture Jesus is able to summarize the whole of the law, and He rightly points out that the commandments pertaining to our relationship with God are of higher import, but not at the exclusion of His commandments regarding our human relationships. The Pharisees, like the Sadducees, have been silenced.
Sometimes, especially as Catholics, we make our faith more complicated than it needs to be. To be sure, our faith is complicated; as CS Lewis writes in Mere Christianity: “It is no good asking for a simple religion. After all, real things are not simple.” But living out our faith need not be complicated if we take to heart the commands of God and the example of Jesus, especially His great commandment which, in a way, sums up the law of God even more succinctly than today’s Gospel: “As I have loved you, so you also should love one another,” (John 13:34).