John Calvin Commentary Deuteronomy 4:9

"Only take heed to thyself, and keep thy soul diligently, lest thou forget the things which thine eyes saw, and lest they depart from thy heart all the days of thy life; but make them known unto thy children and thy children`s children;" — Deuteronomy 4:9 (ASV)

Only take heed to yourself. The same particle, רק, rak, about which I have just spoken, is used here. Its meaning in this place is as if Moses had said that this only remained, unless it is preferred to translate it nevertheless.

What follows means literally, Guard yourself (custodi), and guard your soul. In this, Moses advances by degrees, reminding them that they needed no common heedfulness, but rather that they must beware with extreme vigilance and diligence, lest they should fail for lack of these.

For the slothfulness of the flesh must be spurred on by such promptings as these; at the same time, our weakness must be fortified, and we must take measures against our unsteadfastness. For nothing is easier than for all our zeal to be suddenly forgotten or to gradually grow cold.

God had established the certainty of His law, as far as was necessary, for the grateful and attentive. Yet, not without reason does He desire the people to remember how great is the carelessness of men.

Nor does He command only those who were eyewitnesses to remember, but also to hand down what they had seen to their sons and grandsons, that the memory of such remarkable things might be preserved.