A Commentary, Critical, Practical, and Explanatory on the Old and New Testaments, by Robert Jamieson, A.R. Fausset and David Brown  at sacred-texts.com
deu 6:1MOSES EXHORTS ISRAEL TO HEAR GOD AND TO KEEP HIS COMMANDMENTS. (Deu. 6:1-25)
Now these are the commandments, the statutes, and the judgments, which the Lord your God commanded to teach you, that ye might do them . . . whither ye go to possess it--The grand design of all the institutions prescribed to Israel was to form a religious people, whose national character should be distinguished by that fear of the Lord their God which would ensure their divine observance of His worship and their steadfast obedience to His will. The basis of their religion was an acknowledgment of the unity of God with the understanding and the love of God in the heart (Deu 6:4-5). Compared with the religious creed of all their contemporaries, how sound in principle, how elevated in character, how unlimited in the extent of its moral influence on the heart and habits of the people! Indeed, it is precisely the same basis on which rests the purer and more spiritual form of it which Christianity exhibits (Mat 22:37; Mar 12:30; Luk 10:27). Moreover, to help in keeping a sense of religion in their minds, it was commanded that its great principles should be carried about with them wherever they went, as well as meet their eyes every time they entered their homes. A further provision was made for the earnest inculcation of them on the minds of the young by a system of parental training, which was designed to associate religion with all the most familiar and oft-recurring scenes of domestic life. It is probable that Moses used the phraseology in Deu 6:7 merely in a figurative way, to signify assiduous, earnest, and frequent instruction; and perhaps he meant the metaphorical language in Deu 6:8 to be taken in the same sense also. But as the Israelites interpreted it literally, many writers suppose that a reference was made to a superstitious custom borrowed from the Egyptians, who wore jewels and ornamental trinkets on the forehead and arm, inscribed with certain words and sentences, as amulets to protect them from danger. These, it has been conjectured, Moses intended to supersede by substituting sentences of the law; and so the Hebrews understood him, for they have always considered the wearing of the Tephilim, or frontlets, a permanent obligation. The form was as follows: Four pieces of parchment, inscribed, the first with Exo 13:2-10; the second with Exo 13:11-16; the third with Deu 6:1-8; and the fourth with Deu 11:18-21, were enclosed in a square case or box of tough skin, on the side of which was placed the Hebrew letter (shin), and bound round the forehead with a thong or ribbon. When designed for the arms, those four texts were written on one slip of parchment, which, as well as the ink, was carefully prepared for the purpose. With regard to the other usage supposed to be alluded to, the ancient Egyptians had the lintels and imposts of their doors and gates inscribed with sentences indicative of a favorable omen [WILKINSON]; and this is still the case, for in Egypt and other Mohammedan countries, the front doors of houses (in Cairo, for instance) are painted red, white, and green, bearing conspicuously inscribed upon them such sentences from the Koran, as "God is the Creator," "God is one, and Mohammed is his prophet." Moses designed to turn this ancient and favorite custom to a better account and ordered that, instead of the former superstitious inscriptions, there should be written the words of God, persuading and enjoining the people to hold the laws in perpetual remembrance.
deu 6:20when thy son asketh thee in time to come, saying--The directions given for the instruction of their children form only an extension of the preceding counsels.