Mark 12:28-30 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.
If the LORD was the "only" God, then they ought not to love any other being supremely - then they might not bow down before any idol.
Exodus 20:3-5 Thou shalt have no other gods before me.
Thou shalt not make unto thee any graven image, or any likeness of any thing that is in heaven above, or that is in the earth beneath, or that is in the water under the earth:
Thou shalt not bow down thyself to them, nor serve them: for I the LORD thy God am a jealous God, visiting the iniquity of the fathers upon the children unto the third and fourth generation of them that hate me;
Mark 12:31 And the second is like, namely this, Thou shalt love thy neighbour as thyself. There is none other commandment greater than these.
Regards