You shall love the Lord your God with all your heart and mind and with all your soul and with all your strength [your entire being]. These words, which I am commanding you today, shall be [written] on your heart and mind. You shall teach them diligently to your[b]children [impressing God’s precepts on their minds and penetrating their hearts with His truths] and shall speak of them when you sit in your house and when you walk on the road and when you lie down and when you get up. And you shall bind them as a sign on your hand (forearm), and they shall be used as [c]bands (frontals, frontlets) on your forehead. You shall write them on the [d]doorposts of your house and on your gates. – Deuteronomy 6:5-9
The “heart” is the inner nature of the man, including his intellectual, emotional, and cognitive futurities; the “soul” is the personality, the entire self-consciousness; and the” might” is the sum of the energies, bodily and mental. Not by profession merely is Jehovah to be loved; the whole man, body, soul, and spirit, is to be yielded to him in holy and devout affection (cf.Matthew 22:37; Mark 12:33; Luke 10:27; Romans 12:1). The last letter of the first word, and the last letter of the last word in this verse are larger than the ordinary size (majuscula), and as these two form the word for witness (עד), the Jews say that they are written thus “that every one may know, when he professes the unity of God, that his heart ought to be intent and devoid of every other thought, because God is a witness, and knoweth everything” (R. Bechai, fol. 195, quoted by Michaelis, ‘Bib. Heb,’ in loc.). – Pulpit Commentary
This command seems to be impossible to obey. That’s because, in the natural state of man, it is impossible. There is no greater evidence of man’s inability to obey God’s Law than this one commandment. No human being with a fallen nature can possibly love God with all his heart, soul, and strength 24 hours a day. It’s humanly impossible. But to disobey any commandment of God is sin. Therefore, even without considering the sins we commit daily, we are all condemned by our inability to fulfill this one commandment. This is the reason Jesus continually reminded the Pharisees of their inability to keep the Law of God. He was trying to get them to see their utter spiritual bankruptcy and their need for a Savior. Without the cleansing of sin that He provides, and the empowering presence of the Holy Spirit who lives in the hearts of the redeemed, loving God to any degree is impossible.
While surprising to some, the old covenant recognized that a spiritual relationship with God begins from within, with a proper disposition toward the preeminent saviour, sovereign, and satisfier. From the heart “flow the springs of life” (Proverbs 4:23), and without one’s will, desires, passions, affections, perceptions, and thoughts rightly aligned, the life of love is impossible. Therefore Moses calls Israel to “know . . . in your heart” that God disciplines like a father his son. He urges God’s people to “lay it to heart” that there is no God besides Yahweh (Deuteronomy 4:39–40) and to make sure that His words “be on your heart” (Deuteronomy 6:6), thus anticipating the miraculous heart-work that the new covenant would realize (Jeremiah 31:33).