Deuteronomy 32:1

Here is, I. A commanding preface or introduction to this song of Moses, Deu 32:1, Deu 32:2. He begins, 1. With a solemn appeal to heaven and earth concerning the truth and importance of what he was about to say, and the justice of the divine proceedings against a rebellious and backsliding people, for he had said (Deu 31:28) that he would in this song call heaven and earth to record against them. Heaven and earth would sooner hear than this perverse and unthinking people; for they revolt not from the obedience to their Creator, but continue to this day, according to his ordinances, as his servants (Psa 119:89-91), and therefore will rise up in judgment against rebellious Israel. Heaven and earth will be witnesses against sinners, witnesses of the warning given them and of their refusal to take the warning (see Job 20:27); the heaven shall reveal his iniquity, and the earth shall rise up against him. Or heaven and earth are here put for the inhabitants of both, angels and men; both shall agree to justify God in his proceedings against Israel, and to declare his righteousness, Psa 50:6; see Rev 19:1, Rev 19:2. 2. he begins with a solemn application of what he was about to say to the people (Deu 32:2): My doctrine shall drop as the rain. "It shall be a beating sweeping rain to the rebellious;" so one of the Chaldee paraphrasts expounds the first clause.

— Matthew Henry's Concise Commentary (public domain)