What did Jesus say is the second greatest commandment?

In Matthew 22:37-39 (and Mark 12:29-31), when a teacher of the law asked Jesus which commandment was the greatest, Jesus replied that the greatest commandment was to love the Lord your God with all your heart, soul, and mind. He then added: "And the second is like it: 'Love your neighbour as yourself.'" Jesus said that all the Law and the Prophets hang on these two commandments.

This second commandment comes from Leviticus 19:18 in the Old Testament. By pairing it with the command to love God, Jesus summarised the entire moral law into two relational principles: vertical love (toward God) and horizontal love (toward other people). The command to love one's neighbour as oneself encompasses the ethical dimensions of the Ten Commandments, which deal with human relationships (do not murder, steal, commit adultery, bear false witness, or covet).

While "Honour your parents," "Keep the Sabbath," and "Do not kill" are all genuine commandments from the Decalogue, none of them is what Jesus identified as the second greatest. Those are individual commandments within the broader law; loving your neighbour as yourself is the overarching principle that encompasses them all.