Mark"s account included Deuteronomy 6:4, which Matthew omitted. This verse, the first in the Shema ( Deuteronomy 6:4-5; cf. Deuteronomy 11:13-21; Numbers 15:37-41) that the Jews repeated twice daily, provides a basis for Deuteronomy 6:5. Shema is the first Hebrew word in this passage, and it means "Hear." Matthew"s Jewish readers would have understood this, but Mark"s Gentile readers probably would not have. Mark 12:4 is an affirmation of belief in the unity of God (i.e, in monotheism). Many of Mark"s original readers had formerly been polytheists. [source][source][source]
"God is to be loved completely and totally ( Mark 12:30) because Hebrews , and he alone, is God and because he has made a covenant of love with his people. In the covenant God gives himself totally in love to his people; therefore he expects his people to give themselves totally ("soul," "mind," and "strength") in love to him." [1][source]
"Heart" represents the control center of human personality, "soul" the self-conscious thought life, "mind" the thought capacity, and "strength" all of one"s bodily powers. [2] These are to be the sources out of which love for God should flow. We should love God with all our will (decisions), emotions (desires), minds (thoughts), and bodies (actions). [source][source][source]
"A comparison of the order-heart, soul, mind (Matthew); heart, soul, mind, strength (Mark); heart, soul, strength, mind (Luke); heart, soul, strength (the Masoretic Text); and mind, soul, strength (the Septuagint)-among the various lists suggests that Mark and Luke added "mind" to the Hebrew/Septuagintal formula whereas Matthew substituted "mind" for "strength."" [3][source]