Leviticus 19:1-2, 15-18
1 Thessalonians 2:1-8
According to Matthew, everybody who is anybody in Jerusalem power circles is out to get Jesus. Chief priests, elders, scribes, lawyers, Sadducees, Pharisees--folks who would not necessarily agree with one another about the proper interpretation of the Scriptures--all want to take this teacher to task, to trip him up in questions on the law. If they cannot nail him on political grounds ("Is it lawful to pay taxes to the emperor?"), maybe they can do so on religious ones ("Which commandment in the law is the greatest?").
They could have ignored this character from the backwoods of Galilee. After all, he has no political clout, and he is not preaching violent rebellion, given that he teaches people to love their enemies (Mt 5:44) and turn the other cheek (5:39). But still, there is the matter of all of these people in Jerusalem for the festival, with crowds gathering around Jesus and proclaiming him a prophet (21:9-11). And everybody knows what prophets do, how they rail against religious leaders, challenging the status quo. A prophet in the city during festival-time is certainly a nuisance, probably a problem, and possibly a danger.
It must have been frustrating, the way that Jesus bested his opponents at every turn. When they question his authority (Mt 21:23ff.) he rebuffs them with riddles (The Two Sons, The Wicked Tenants, The Wedding Banquet). When they challenge him directly (22:17,23:24-28) he responds with enigma. Whenever they try to defeat him with the Torah, they end up feeling defeated themselves. Tired of watching this Galilean overcome his opponents at every turn, the Pharisees take matters into their own hands by sending one of their best--a professionally trained theologian ("lawyer")--to test Jesus with the ultimate rabbinic question: "Which commandment in the law is the greatest?"
The question is important to the Pharisees. A relatively small Jewish sect at the time of Jesus, Pharisaic Judaism rose in prominence after the destruction of the Temple in 70 C.E. The notoriety of the Pharisees in the Gospel narratives, as well as the polemic against them, probably reflects the authors' own time more than it does the historical Jesus. In any case, questions about the law were at the heart of Pharisaic Judaism, particularly questions about how--exactly--one might best love God by one's adherence in daily life to the commands of Torah. They did not have to agree...