This is one of several tests different religious groups used to try and trap Jesus (Mark 12:13):
And one of the scribes came up and heard them disputing with one another, and seeing that he answered them well, asked him, “Which commandment is the most important of all?”
(Mark 12:28) [ESV]
This test came from a scribe, a "γραμματεύς." Scribes were men learned in the Mosaic law and in the sacred writings, an interpreter, teacher who examined the more difficult and subtle questions of the law; added to the Mosaic law decisions of various kinds thought to elucidate its meaning and scope, and did this to the detriment of religion.
1 It is important to remember this context: the scribe is trying to trap Jesus. In other words, if the answer was obvious, there is no trap. Here is the complete answer Jesus gave:
29 Jesus answered, “The most important is, ‘Hear, O Israel: The Lord our God, the Lord is one. 30 And you shall love the Lord your God with all your heart and with all your soul and with all your mind and with all your strength.’ 31 The second is this: ‘You shall love your neighbor as yourself.’ There is no other commandment greater than these.” (Mark 12)
The first part of the answer Jesus gives is taken from Deuteronomy:
1 “Now this is the commandment—the statutes and the rules—that the LORD your God commanded me to teach you, that you may do them in the land to which you are going over, to possess it, 2 that you may fear the LORD your God, you and your son and your son's son, by keeping all his statutes and his commandments, which I command you, all the days of your life, and that your days may be long. 3 Hear therefore, O Israel, and be careful to do them, that it may go well with you, and that you may multiply greatly, as the LORD, the God of your fathers, has promised you, in a land flowing with milk and honey. 4 “Hear, O Israel: The LORD our God, the LORD is one. 5 You shall love the LORD your God with all your heart and with all your soul and with all your might. 6 And these words that I command you today shall be on your heart. (Deuteronomy 6)
The essence of the scribe's "trap" is the relationship of the Shema to the Torah.
The greatest command is not one of the Ten Commandments (Deuteronomy 5:1-21). It was not spoken directly from the LORD to the people; nor was it written on the two stone tablets (Deuteronomy 5:22). It was only given after the people became terrified of the LORD and perhaps most significantly, it was not given to those who were brought out of Egypt (Deuteronomy 5:23-33). How can one claim the greatest commandment is something which was not given to those who actually experienced the LORD's deliverance?
Jesus avoided the trap by saying the greatest command was actually composed of two different commands which could still be called a single command. The second part is taken from Leviticus:
You shall not take vengeance or bear a grudge against the sons of your own people, but you shall love your neighbor as yourself: I am the LORD. (Leviticus 19:18)
Of this section of Leviticus Baruch J. Schwartz says:
...there is no such things as a command pertaining merely to relations between human beings...every commanded or prohibited action affects the sacred realm and is in the category of laws between God and human beings...The chapter expresses this by presenting an admixture of laws from every sphere - from worship to fairness in commerce, from legal proceedings to reverence for the Temple, from idolatry and the avoidance of pagan practices to family relations, from the use of the name of God in oaths to support for the needy, from the sanctity of first fruits to theft and fraud - all on equal footing; punctuating the separate paragraphs with the repeated refrain I am the LORD or I the LORD am your God, which is approximately equivalent to "because I, the LORD, say so." The opening and closing vv. (2 and 37) provide the key to meaning: You shall be holy (v. 2). You shall faithfully observe all My laws and all My rules: I am the LORD (v. 37).
This command was given to those who were brought out of Egypt. While it was the second one cited by Jesus, it was the first one given and there is a connection to the Shema:
Only through faithful observance of God's commands can the Israelite fulfill the sacred charge of being holy. This is repeated in Num. 15:39-40, recited every morning and evening as part of the Shema prayer...
The Shema prayer is composed of three parts:
Shema Prayer Historical "giving" of the prayer
Part 1: Deuteronomy 6:4-9 Numbers 15:39-40
Part 2: Deuteronomy 11:13-21 Deuteronomy 6:4-9
Part 3: Numbers 15:39-40 Deuteronomy 11:13-21
Had it been in effect, the only portion of the morning and evening Shema prayer which those who were brought out of Egypt could recite is from Numbers:
39 And it shall be a tassel for you to look at and remember all the commandments of the LORD, to do them, not to follow after your own heart and your own eyes, which you are inclined to whore after. 40 So you shall remember and do all my commandments, and be holy to your God. 41 I am the LORD your God, who brought you out of the land of Egypt to be your God: I am the LORD your God.” (Numbers 15)
Therefore, the answer Jesus gave is actually a summary to the Shema prayer:
Shema Prayer Greatest command "now" Greatest command "then"
Part 1: Deuteronomy 6:4-9 Deuteronomy 6:4-5 Leviticus 19:18
Part 2: Deuteronomy 11:13-21 Deuteronomy 6:4-5
Part 3: Numbers 15:39-40 Leviticus 19:18
Both the Shema and the Scripture Jesus used have similar ending: I am the LORD, or as Schwartz says: Because I the LORD say so. Therefore, historically Deuteronomy 6:4-5 was appended to the greatest command which initially was simply: Love your neighbor as yourself. Yet "now" begins: And you shall love the LORD your God with all your heart and all your soul and with all your mind and with all your strength. Why? Because I the LORD say so.
The scribe acknowledged the correctness of Jesus handling of the Law:
32 And the scribe said to him, “You are right, Teacher. You have truly said that he is one, and there is no other besides him. 33 And to love him with all the heart and with all the understanding and with all the strength, and to love one's neighbor as oneself, is much more than all whole burnt offerings and sacrifices.” (Mark 12)
And then Jesus made this statement to the scribe:
And when Jesus saw that he answered wisely, he said to him, “You are not far from the kingdom of God.” And after that no one dared to ask him any more questions. (Mark 12:34)
Despite correctly embellishing Jesus' answer by including the inefficacy of animal sacrifice, the scribe was not far from the Kingdom of God. That is, he was close, but there was still something he was missing.
There were two things the scribe failed to grasp. First, he was not a disciple. Jesus gave a good answer, but the scribe's response was to give Jesus a "legal brief" not to become a follower. Secondly, Jesus had not yet made the final sacrifice by which those were were "far off or near" could gain entry to the Kingdom:
But now in Christ Jesus you who once were far off have been brought near by the blood of Christ. (Ephesians 2:13)
And he came and preached peace to you who were far off and peace to those who were near. For through him we both have access in one Spirit to the Father. (Ephesians 2:17-18)
Until Jesus gave up His life even those who were near were like those who were far off.
Who is the one Lord we should love?
9 As the Father has loved me, so have I loved you. Abide in my love. 10 If you keep my commandments, you will abide in my love, just as I have kept my Father's commandments and abide in his love. (John 15)
By abiding in Jesus' love, His disciples abide in the Father's love. Moreover, Jesus replaced the second part of the greatest command:
A new commandment I give to you, that you love one another: just as I have loved you, you also are to love one another. (John 13:33)
Just like the Israelites who were brought out of Egypt were not given the Shema, but were told to love others as self, the disciples are given a law of love to others. But the standard is much higher: love of self is replaced by Jesus' love. No longer will loving others as one loves self suffice. No! Now one is to love others the way Jesus loved us, which is to say, He gave His life for us.
The God we are to love is the one who commands us to love, not as we love ourselves, but as He loves us.
2. Baruch J. Schwartz, The Jewish Study Bible, Edited by Adele Berlin and Marc Zvi Brettler, Oxford University Press, 2004, p. 252