Shall be in thine heart - For where else can love be? If it be not in the heart, it exists not. And if these words be not in the heart - if they are not esteemed, prized, and received as a high and most glorious privilege, what hope is there that this love shall ever reign there?

And these words, which I command thee this day,.... To hearken to, observe, and take notice of, that God is one, and is to be loved in the strongest manner that possibly can be:

shall be in thine heart; on the table of the heart, as the Targum of Jonathan; see 2 Corinthians 3:3, be cordially received, have a place in the affections of the heart, and be retained in mind and memory.

But for the love of God to be of the right kind, the commandments of God must be laid to heart, and be the constant subject of thought and conversation. "Upon thine heart:" i.e., the commandments of God were to be an affair of the heart, and not merely of the memory (cf. Deuteronomy 11:18). They were to be enforced upon the children, talked of at home and by the way, in the evening on lying down and in the morning on rising up, i.e., everywhere and at all times; they were to be bound upon the hand for a sign, and worn as bands (frontlets) between the eyes (see at Exodus 13:16). As these words are figurative, and denote an undeviating observance of the divine commands, so also the commandment which follows, viz., to write the words upon the door-posts of the house, and also upon the gates, are to be understood spiritually; and the literal fulfilment of such a command could only be a praiseworthy custom or well-pleasing to God when resorted to as the means of keeping the commandments of God constantly before the eye. The precept itself, however, presupposes the existence of this custom, which is not only met with in the Mahometan countries of the East at the present day (cf. A. Russell, Naturgesch. v. Aleppo, i. p. 36; Lane, Sitten u. Gebr. i. pp. 6, 13, ii. p. 71), but was also a common custom in ancient Egypt (cf. Wilkinson, Manners and Customs, vol. ii. p. 102).

(Note: The Jewish custom of the Medusah is nothing but a formal and outward observance founded upon this command. It consists in writing the words of Deuteronomy 6:4-9 and Deuteronomy 11:13-20 upon a piece of parchment, which is then placed upon the top of the doorway of houses and rooms, enclosed in a wooden box; this box they touch with the finger and then kiss the finger on going either out or in. S. Buxtorf, Synag. Jud. pp. 582ff.; and Bodenschatz. Kirchl. Verfassung der Juden, iv. pp. 19ff.)

And these words, which I command thee this day, shall be in thine heart:

6:6-16 Here are means for maintaining and keeping up religion in our hearts and houses. 1. Meditation. God's words must be laid up in our hearts, that our thoughts may be daily employed about them. 2. The religious education of children. Often repeat these things to them. Be careful and exact in teaching thy children. Teach these truths to all who are any way under thy care. 3. Pious discourse. Thou shalt talk of these things with due reverence and seriousness, for the benefit not only of thy children, but of thy servants, thy friends and companions. Take all occasions to discourse with those about thee, not of matters of doubtful disputation, but of the plain truths and laws of God, and the things that belong to our peace. 4. Frequent reading of the word. God appointed them to write sentences of the law upon their walls, and in scrolls of parchment to be worn about their wrists. This seems to have been binding in the letter of it to the Jews, as it is to us in the intent of it; which is, that we should by all means make the word of God familiar to us; that we may have it ready to use upon all occasions, to restrain us from sin, and direct us in duty. We must never be ashamed to own our religion, nor to own ourselves under its check and government. Here is a caution not to forget God in a day of prosperity and plenty. When they came easily by the gift, they would be apt to grow secure, and unmindful of the Giver. Therefore be careful, when thou liest safe and soft, lest thou forget the Lord. When the world smiles, we are apt to make court to it, and expect our happiness in it, and so we forget Him who is our only portion and rest. There is need of great care and caution at such a time. Then beware; being warned of your danger, stand upon your guard. Thou shalt not tempt the Lord thy God; neither by despairing of his power and goodness, while we keep in the way of our duty; nor by presuming upon it, when we turn aside out of that way.