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The Great Commandment (or Greatest Commandment) is a term used in the New Testament to describe the first of two commandments cited by Jesus in Matthew 22:35–40, Mark 12:28–34. These two commandments are taken from the Law of Moses in the Old Testament and are commonly seen as important to Christian ethics.
In Matthew, when asked "which is the great commandment in the law?", the Bible reports that Jesus answered, "Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind", before also referring to a second commandment, "Thou shalt love thy neighbour as thyself". Most Christian denominations consider these two commandments the core of the Christian religion.
Then one of them, which was a lawyer, asked him a question, tempting him, and saying, Master, which is the great commandment in the law? Jesus said unto him, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind. This is the first and great commandment. And the second is like unto it, Thou shalt love thy neighbour as thyself. On these two commandments hang all the law and the prophets.
In the Gospel of Mark, the Shema is included:
And one of the scribes came, and having heard them reasoning together, and perceiving that he had answered them well, asked him, Which is the first commandment of all? And Jesus answered him, The first of all the commandments is, Hear, O Israel; The Lord our God is one Lord: And thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind, and with all thy strength: this is the first commandment. And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.
And, behold, a certain lawyer stood up, and tempted him, saying, Master, what shall I do to inherit eternal life? He said unto him, What is written in the law? how readest thou? And he answering said, Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy strength, and with all thy mind; and thy neighbour as thyself. And he said unto him, Thou hast answered right: this do, and thou shalt live.
Hear, O Israel: The LORD our God is one LORD: And thou shalt love the LORD thy God with all thine heart, and with all thy soul, and with all thy might.
Thou shalt not avenge, nor bear any grudge against the children of thy people, but thou shalt love thy neighbour as thyself: I am the LORD.
Matthew Henry sums up the question of which is the great commandment,
It was a question disputed among the critics in the law. Some would have the law of circumcision to be the great commandment, others the law of the sabbath, others the law of sacrifices, according as they severally stood affected, and spent their zeal; now they would try what Christ said to this question, hoping to incense the people against him, if he should not answer according to the vulgar opinion; and if he should magnify one commandment, they would reflect on him as vilifying the rest.
This is the first and great commandment - It is so,
1. In its antiquity, being as old as the world, and engraven originally on our very nature.
2. In dignity; as directly and immediately proceeding from and referring to God.
3. In excellence; being the commandment of the new covenant, and the very spirit of the Divine adoption.
4. In justice; because it alone renders to God his due, prefers him before all things, and secures to him his proper rank in relation to them.
5. In sufficiency; being in itself capable of making men holy in this life, and happy in the other.
6. In fruitfulness; because it is the root of all commandments, and the fulfilling of the law.
7. In virtue and efficacy; because by this alone God reigns in the heart of man, and man is united to God.
8. In extent; leaving nothing to the creature, which it does not refer to the Creator.
9. In necessity; being absolutely indispensable.
10. In duration; being ever to be continued on earth, and never to be discontinued in heaven.
"Thou shalt love the Lord thy God" is explained to mean "Act in such a manner that God will be beloved by all His creatures." Consequently Israel, being, as the priest-people, enjoined like the Aaronite priest to sanctify the name of God and avoid whatever tends to desecrate it (Lev. xxii. 32), is not only obliged to give his life as witness or martyr for the maintenance of the true faith (see Isa. xliii. 12, μάρτυρες; and Pesik. 102b; Sifra, Emor, ix.), but so to conduct himself in every way as to prevent the name of God from being dishonored by non-Israelites.
Twice every day the Jew recites the Shema, which contains the words: "Thou shalt love the Lord thy God with all thine heart, with all thy soul, and with all thy might" (Deut. vi. 5). This verse is understood to enjoin him to willingly surrender life and fortune whenever the cause of God demands it, while it at the same time urges him to make God beloved by all his creatures through deeds of kindness, as Abraham did (Sifre, Deut. 32).
Although only asked about the first commandment, Jesus included the second commandment in his answer – This double reference has given rise to differing views with regard to the relationship that exists between the two commandments, although typically "love thy God" is referred to as "the first and greatest commandment", with "love thy neighbour" being referred to as "the second great commandment".
Hillel the Elder, an elder contemporary of Jesus, formulated a negative form of the Golden Rule and when asked to sum up the entire Torah concisely to a gentile who wished to become a Jew, he answered, "What is hateful to you, do not do to your fellow: this is the whole Law; the rest is the explanation."
With these words Hillel recognized as the fundamental principle of the Jewish moral law the biblical precept of brotherly love (Lev. xix. 18). Almost the same thing was taught by Paul, a [former] pupil of Gamaliel, the grandson of Hillel (Gal. v. 14; comp. Rom. xiii. 8); and more broadly by Jesus when he declared the love of one's neighbour to be the second commandment beside the love of God, the first (Matt. xxii. 39; Mark xii. 31; Luke x. 27). Akiva ben Joseph, a tanna of first and second century Judaism, called "Thou shalt love thy neighbor as thyself" the "a great principle of Judaism".
C.S. Lewis also offers a substantial commentary on loving your neighbor. In his works, "Mere Christianity" He says "This is what is meant in the Bible by loving him: wishing his good, not feeling fond of him nor saying he is nice when he is not." 
"For, brethren, ye have been called unto liberty; only use not liberty for an occasion to the flesh, but by love serve one another. For all the law is fulfilled in one word, even in this; Thou shalt love thy neighbour as thyself. But if ye bite and devour one another, take heed that ye be not consumed one of another."
The Gospel of Luke connects a story (Luke 10:25-28) similar to the Great Commandment stories of Mark and Matthew with a unique account of a good Samaritan (Luke 10:29-37) told to illustrate who a person's "neighbour" was. The story involves a stranger befriending and aiding a beaten man who had been overlooked or ignored by others passing by. After relating the story, Jesus instructed the questioner to "Go, and do thou likewise."
Brotherly love in the biblical sense is an extension of the natural affection associated with near kin, toward the greater community of fellow believers, that goes beyond the mere duty in Leviticus 19:18 to "love thy neighbour as thyself", and shows itself as "unfeigned love" from a "pure heart", that extends an unconditional hand of friendship that loves when not loved back, that gives without getting, and that ever looks for what is best in others.