Having completed his discourse on the laws that should govern Israel, Moses reminds Israel of the necessity of obedience: obedience to these laws is obedience to God. Moses warns the Israelites: Do not obey the law as if it were a mere legal code instead show your love of God by obeying.
"This day the LORD your God commands you to do these statutes and ordinances; you shall therefore be careful to do them with all your heart and with all your soul. You have declared this day concerning the LORD that he is your God and that you will walk in his ways, and keep his statutes and his commandments and his ordinances, and will obey his voice; and the LORD has declared this day concerning you that you are a people for his own possession, as he has promised you, and that you are to keep all his commandments." Deut. 26:16-18
In these three verses, we see that, in Moses's eyes, the law is not mere legality; it is the basis of a marital covenant between God and his people, Israel. This bond is everlasting for God's word will never be broken, and if everlasting, then the source of principle of life for Israel. Thus, idolatry is spiritual adultery.
Moses's words are not meant for Israel only. They are even more applicable for the Church, the New Israel. By studying his words we gain a greater appreciation of the mysterious relation between Christ and his Church.