Matthew 22:34-40

34 But when the Pharisees heard that he had silenced the Sadducees, they gathered together. 35 And one of them, a lawyer, asked him a question to test him. 36 “Teacher, which is the great commandment in the Law?” 37 And he said to him, “You shall love the Lord your God with all your heart and with all your soul and with all your mind. 38 This is the great and first commandment. 39 And a second is like it: You shall love your neighbor as yourself. 40 On these two commandments depend all the Law and the Prophets.”

Everything in the Old Testament is about love for God and love for neighbor, and everything Jesus teaches in Matthew is based on that Great Commandment.