Charles Ellicott 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 exhortation contained in the following verses lays special emphasis on one point—the worship of the invisible Jehovah without images. This more than anything else would tend to separate the religion of Israel from that of all other nations.

Teach them your sons, and your sons’ sons. —A command which Israel evidently failed to obey. For a generation quickly rose up which knew not Jehovah nor yet the works which he had done for Israel (Judges 2:10).

It is worthwhile to observe that we cannot find any trace of a system of national education in Israel until many years later. When education is purely parental, it is likely to be neglected in many instances. It is not every parent who finds themselves able to teach their sons, and their sons’ sons.