Teacher, which commandment in the Law is the greatest? It appears that the religious leaders were really determined to remove Jesus from their midst. Even after failing to have Jesus say something that would land him in trouble with the Roman government (the question on paying taxes to Caesar [22:15-22]) or have Jesus entangle himself in speech (the question about the resurrection [22:23-33]), they refused to back down. They remained hopeful that Jesus would have a ‘slip of the tongue’ and make it easy for them to bring accusations against him. In today’s Gospel reading, the Pharisees decide to up their game. They had for long doubted Jesus’ authenticity as a rabbi (cf. Matthew 21:23-27). Anyone who would continuously show a disregard for the laws and traditions of his community (healing on the Sabbath, healing within the confines of the sacred places of worship, letting his disciples pick corn on the Sabbath, and making blasphemous statements such as calling God his father and telling people their sins were forgiven) could not be a real rabbi. And so, they put forth to Jesus a question that a true son of his community was expected to be familiar with: the question about the greatest commandment. 
The greatest (first) commandment was part of the  Shema prayer (‘Hear O Israel…’) ,the prayer that was the centerpiece of the daily morning and evening prayer of the Jews. It was a prayer that was to be taught by the fathers to their children and made the central piece of their lives (cf. Deuteronomy 6:4-9). Of course, Jesus knew the prayer and he gave them a perfect answer. But Jesus went beyond the extent of the question put to him and joined to the greatest and first commandment a second one, that which deals with one’s neighbor. While this combination of the two commandments may already have been put in place in the practice of their religion (cf. Leviticus 19:18), its emphasis by Jesus was not without a reason. 
Jesus knew why the Pharisees had asked him about the greatest commandment. But he also knew that many in the crowd had gathered to listen to his teaching and as such were genuinely interested in how he was answering the question. And so, it was to their benefit that he joined the second commandment to the first. He was aware that those in the crowd knew what the greatest commandment said. However, they might not have paid much attention to how it was to be applied in real life. The ‘second’ commandment that he provided was the expression or practical application of the ‘first’ one. As St. John reminds us, God can only be loved by loving one’s brothers and sisters (cf. I John 4:20). The command to love God could not be separated from the command to love one’s neighbor for the one (loving God) is realized in the other (loving the neighbor). The command to love God and the command to love one’s neighbor are complimentary and the one cannot be observed without the other. This was not something new that Jesus was saying. It was right there as part of the tradition of the community. It had simply been ignored or disregarded by those whose responsibility it was to teach the law.
Although to the Jewish people the term “neighbor” was restricted to “fellow countrymen,” Jesus in his teachings used “neighbor” to mean all people, especially the vulnerable ones (foreigners, widows, orphans, the poor). In joining the second commandment to the first, Jesus was reminding the religious leaders that it was not enough to have knowledge of the law. It was equally important to apply that law to their daily living. Jesus had observed how the religious leaders related to their ‘neighbors.’ He had observed how the poor and the vulnerable were not taken care of. Yet, these were the very people whom God specifically commanded to be loved. The command to love the poor and vulnerable by acting charitably towards them was enjoined by God to the people of the covenant as a way of showing gratitude to God. For God had acted with charity and compassion towards them when they were suffering as aliens in a foreign land (cf. Deuteronomy 10:19).