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.
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.
Jesus ... combines all duties (1) in one universal rule (which includes within itself both the inner and the outer moral relations of men), namely: Perform your duty for no motive other than unconditioned esteem for duty itself, i.e., love God (the Legislator of all duties) above all else; and (2) in a particular rule, that, namely, which concerns man’s external relation to other men as universal duty: Love every one as yourself, i.e., further his welfare from good-will that is immediate and not derived from motives of self-advantage. These commands are not mere laws of virtue but precepts of holiness which we ought to pursue, and the very pursuit of them is called virtue.
Immanuel Kant, Religion within the Limits of Reason Alone, Book IV, Part 1, Section 1, “The Christian religion as a natural religion”