Matthew 22
36 Master, which [is] the great commandment in the law? 37 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. 38 This is the first and great commandment. 39 And the second [is] like unto it, Thou shalt love thy neighbour as thyself. 40 On these two commandments hang all the law and the prophets.
Mark 12
28 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? 29 And Jesus answered him, The first of all the commandments [is], Hear, O Israel; The Lord our God is one Lord: 30 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. 31 And the second [is] like, [namely] this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.
The greatest commandment is not, "Hear, O Israel; The Lord our God is one Lord." That is just the preface to it. Matthew 22 omits that part, which tells us that the greatest commandment is, "Thou shalt love the Lord thy God with all thy heart, and with all thy soul, and with all thy mind.."
It is interesting how later in Mark 12, Jesus said this though:
Mark 12
35 And Jesus answered and said, while he taught in the temple, How say the scribes that Christ is the Son of David? 36 For David himself said by the Holy Ghost, The LORD said to my Lord, Sit thou on my right hand, till I make thine enemies thy footstool. 37 David therefore himself calleth him Lord; and whence is he [then] his son? And the common people heard him gladly.
The LORD (one person) said to my Lord (another person), Sit on my right hand, till I make thine enemies thy footstool. The bible is not trying to trick us when it uses this kind of language. There are really two persons interacting with each other here in these verses, as well as in other places
Travis.