IN THOSE DAYS, a certain lawyer came to Jesus asked Him a question to test Him, “Teacher, which commandment in the Law is the greatest?” Jesus said to him, “‘Love the Lord your God with all your heart, and with all your soul, and with all your mind’ (Deuteronomy 6:5). This is the first and greatest commandment. And a second is like it, ‘Love your neighbor as yourself’ (Leviticus 19:18). On these two commandments depend the entire law and the prophets.” Now while the Pharisees were gathered together, Jesus asked them this question, “What do you think of the Messiah? Whose Son is He?” They answered, “The Son of David.” Jesus said to them, “How is it then that David in the Spirit calls Him Lord, saying, ‘The Lord said to My Lord, “Sit at My right hand, until I make Your enemies Your footstool”’ (Psalm 110:1)? If David then calls Him Lord, how can He be his son?” And no one was able to give Jesus an answer, nor from that day did anyone dare to ask Him any more questions.
Saint Cyril of Alexandria explains why these are the greatest commandments,
Therefore, the first commandment teaches every kind of godliness. For to love God with the whole heart is the cause of every good. The second commandment includes the righteous acts we do towards other people. The first commandment prepares the way for the second and in turn is established by the second. For the person who is grounded in the love of God clearly also loves his neighbor in all things himself. The kind of person who fulfills these two commandments experiences all the commandments.
The Whole Counsel of God Bible study: