One of the teachers of the law came and heard them debating. Noticing that Jesus had given them a good answer, he asked him, "Of all the commandments, which is the most important?"
"The most important one," answered Jesus, "is this: 'Hear, O Israel: The Lord our God, the Lord is one. Love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.' The second is this: 'Love your neighbor as yourself.' There is no commandment greater than these."
"Well said, teacher," the man replied. "You are right in saying that God is one and there is no other but him. To love him with all your heart, with all your understanding and with all your strength, and to love your neighbor as yourself is more important than all burnt offerings and sacrifices."
When Jesus saw that he had answered wisely, he said to him, "You are not far from the kingdom of God." And from then on no one dared ask him any more questions.
Mark 12:28–34 occurs during the last week before the crucifixion. Jesus spends time in the temple courtyard, teaching the people and debating Jewish religious and civil leaders. Intrigued by how Jesus proves the resurrection of the dead to a group of Sadducees (Mark 12:18–26), a scribe of the Pharisees (Matthew 22:34–35) asks Jesus about the greatest commandment in the Mosaic law. The central idea of Jesus' answer is to love God and love others. But He starts at the beginning of the Shema prayer: acknowledge God is your God and He is one. This story is also in Matthew 22:34–40.
"Love" here doesn't just mean to have affections for. It is from the Greek root word agapao, which many are more familiar with through the noun agape. To agape-love someone is a choice. It is to choose to see them in a favorable light. To choose to delight in them. And to choose to faithfully act on their behalf. When we agape-love God, we see Him as honorable, true, and loving. We trust Him and respond to that trust with obedience.
Jesus adds more nuance to this love. "Heart" is from the Greek root word kardia, and means the center of one's life, whether physical, spiritual, or mental. The heart drives one's beliefs, emotions, thoughts, and intentions. "Soul" is from the Greek root word psychē which is similar to the heart, but focuses on the vital force that keeps us alive. "Mind" is not found in the original Old Testament quote (Deuteronomy 6:5), but is certainly appropriate to add for an audience influenced by more logical Greek culture. It comes from the Greek root word dianoia and, again, has a similar definition to heart but with a more mental bent.
So, "heart…soul…and mind" cover every aspect of our personalities, being, and thoughts, and everything which influences our feelings, beliefs, desires, and intentions.
"Strength" is a little different. It is from the Greek root word ischyos, and means force, power, and ability. The original Hebrew is from the root word me'ōde, which is also translated as "greatly, exceedingly, and diligently." This "strength," also translated into English as "might," is the effort and passion with which we love God. Where the heart, soul, and mind direct how we feel and what we do, might is the force that determines the extent.
We're not exactly sure of the scribe's intent, here. Unlike the previous two challenges, there's no clear "trap" involved (Mark 12:13). He might simply be asking a deliberately provocative question, expecting Jesus to pick one of the many commandments in the Mosaic law and declare it the greatest. What Jesus has done is give a commandment that encompasses the first four of the Ten Commandments, all of which concern how we interact with God. In the third part of Jesus' answer, to love our neighbors as ourselves, He brings in the last six of the Ten Commandments. In doing so, He reveals that God's laws are not meant to control us, they are meant to describe what it means to love.
Jesus' three-part law—know God, love God, love others—isn't just the first law we need to consider, it encompasses everything God wants us to do.