"The most important (commandment) is, 'Hear, O Israel: Adonai is our God, Adonai is one."
Yeshua just dumb-founded Sadducee priests on the subject of the resurrection of the dead, so some impressed Pharisees (not all were antagonistic) came and asked the Master, "Which commandment is the most important of all (Mark 12:28)?"
Right away we must conclude from this question that not all commandments are equal in importance; some have more, some have less. This can be concluded by the fact that the breaking of some commandments only require an immersion (like eating something not kosher), while some invoke the death penalty (like murder, idolatry, adultery, and breaking the Sabbath). Yeshua agreed to the idea of least and great commandments in, whoever relaxes one of the least of these commandments and teaches others to do the same will be called least in the kingdom of heaven (Matthew 5:19).
To this simple question, Yeshua our Master answered by quoting what has been agreed by Jewish sages for centuries before His manifestation on earth, as being the 'creed' of Judaism, "The most important is, 'Hear, O Israel: The Lord our God, the Lord is one". (Mark 12:29; Deuteronomy 6:4)."
The word 'hear' is this case, as it is also in English, is not solely related to a sensorial experience but rather means 'to hear with the intention to obey'. In most religions, even in Christianity, the theory of God is the primary thing, but in the Torah the primary thing is not to define or quantify God as usual creeds and statements of faith do, but it is our actions and in this case to: 'hear and obey' to the belief and idea that God is the One and Only, and that there is no other before, after, or beside Him. Unless you do that first, there is no point in going any further. It is interesting to realize that from early forms of polytheistic religions to modern Western religions mixed with elements of Greco-Roman paganism and New-Age, this is the elementary truth that the Satan (cursed be him) has fought the most, because once we throw that out, then he has a chance.
Yeshua then continues quoting Moses' words in Deuteronomy, words which teach how we express that belief and the first expression of this obedience is: 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 (Mark 12:30).
After that Yeshua adds His own 'halacha', or application to this commandment and says, "The second is this: 'You shall love your neighbor as yourself.' There is no other commandment greater than these (Mark 12:31)."
In essence we are taught here that the expression of our belief in the singleness of the Creator is to love Him first and foremost with all our hearts (our emotions), with all our souls (our life in this body), and with all our might (our substance) (Deuteronomy 6:5), and that the expression of this love is to love our neighbor as ourselves.