Jamieson, Fausset and Brown on Deuteronomy 32:4

4. He is the Rock--a word expressive of power and stability. The application of it in this passage is to declare that God had been true to His covenant with their fathers and them. Nothing that He had promised had failed; so that if their national experience had been painfully checkered by severe and protracted trials, notwithstanding the brightest promises, that result was traceable to their own undutiful and perverse conduct; not to any vacillation or unfaithfulness on the part of God (Jas 1:17), whose procedure was marked by justice and judgment, whether they had been exalted to prosperity or plunged into the depths of affliction.