Deuteronomy 32:47
For it is not a vain thing for you; because it is your life: and through this thing ye shall prolong your days in the land, whither ye go over Jordan to possess it. For it is not a vain thing for you; because it is your life: and through this thing ye shall prolong your days in the land, whither ye go over Jordan to possess it.
God's words, law and commandments are not a vain thing — not empty, useless, futile, a mere form, or a matter of little consequence. They are Israel's life and the people's life, essential to life and given as safeguards. Through this thing means through obedience to the law.
Through such obedience Israel would prolong its days and life in the land across the Jordan that it was going over to possess, with sustained life and promised blessing in the land. The law gives wisdom and understanding, and its principles underlie true prosperity for nations and individuals. The blessings promised to Israel through obedience rest on the same conditions for every nation and individual and are available to all to the same degree.
Obedience brings lasting well-being, health and prosperity, closely relating keeping God's commandments to well-being in body and spirit. Obedience would preserve Israel from the evil practices of the nations that did not know the true God. Obedience keeps people within God's protection, while breaking His laws exposes them to Satan's attacks and lets the enemy work through their minds.
Obedience would have brought happiness and peace, formed characters and principles of great worth, and produced sanctified love and beauty of character, so that the world would have been filled with holiness and purity. The verse charges parents to teach God's law, and disregard of it, especially the Sabbath, is rebellion against God. To treat faith as if it were not a dominating power and its distinctive features as of little consequence is to turn from His commandments and dishonor God, denying that He is God and that His people are His people.
Keeping covenant relation with Him by conscientiously honoring Him, taking a decided stand for truth and righteousness, and keeping His commandments including the true Sabbath as the sign distinguishing those who serve God from those who serve Him not brings His blessings, which are essential for the people who are to be severely tried. Proclaiming God's message is no empty or useless task but is life, and God's truth is powerful to convict sinners.