Our enemies - i. e., the enemies of Moses and the faithful Israelites; the pagan, more especially those with whom Israel was brought into collision, whom Israel was commissioned to "chase," but to whom, as a punishment for faithlessness, Israel was "sold," Deuteronomy 32:30. Moses leaves the decision, whether "their rock" (i. e. the false gods of the pagan to which the apostate Israelites had fallen away) or "our Rock" is superior, to be determined by the unbelievers themselves. For example, see Exodus 14:25; Numbers. 23; 24; Joshua 2:9 ff; 1-Samuel 4:8; 1-Samuel 5:7 ff; 1-Kings 20:28. That the pagan should thus be constrained to bear witness to the supremacy of Israel's God heightened the folly of Israel's apostasy.

For their rock - The gods and pretended protectors of the Romans.
Is not as our Rock - Have neither power nor influence like our God.
Our enemies themselves being judges - For they often acknowledged the irresistible power of that God who fought for Israel. See Exodus 14:25; Numbers 23:8-12, Numbers 23:19-21; 1-Samuel 4:8.
There is a passage in Virgil, Eclog. iv., ver. 58, very similar to this saying of Moses: -
Pan Deus Arcadia mecum si judice certet,
Pan etiam Arcadia dicat se judice victum.
"Should the god Pan contend with me," (in singing
the praises of the future hero, the deliverer,
prophesied of in the Sibylline books), "were even
Arcadia judge, Pan would acknowledge himself to be
vanquished, Arcadia herself being judge."

For their rock is not as our rock,.... That is, the gods of the Heathens, the rock in which they trusted, are not like the God of Israel, the rock of salvation, in which all true believers, whether Jews or Gentiles, place their confidence; and indeed let that be what it will, that is short of Christ the rock, men lay the stress of their salvation on, it is no rock, but sand, and will stand them in no stead; see Matthew 7:24,
even our enemies themselves being judges; as has been confessed of the God of Israel by the Heathens; see Exodus 14:25; and was by Titus with respect to the destruction of Jerusalem; See Gill on Deuteronomy 32:27; and by the Roman emperors when conquered by the Christians, who asked pardon of the God of the Christians, and owned that the God of Constantine was the true God; See Gill on Revelation 6:16.

The giving up of Israel into the power of the heathen arose, not from the superior power of the heathen and their gods, but solely from the apostasy of Israel from its own God. "Our rock," as Moses calls the Lord, identifying himself with the nation, is not as their rock, i.e., the gods in whom the heathen trust. That the pronoun in "their rock" refers to the heathen, is so perfectly obvious from the antithesis "our rock," that there cannot possibly be any doubt about it. The second hemistich in Deuteronomy 32:30 contains a circumstantial clause, introduced to strengthen the thought which precedes it. The heathen themselves could be arbitrators (vid., Exodus 21:22), and decide whether the gods of the heathen were not powerless before the God of Israel. "Having experience so often the formidable might of God, they knew for a certainty that the God of Israel was very different from their own idols" (Calvin). The objection offered by Schultz, namely, that "the heathen would not admit that their idols were inferior to Jehovah, and actually denied this at the time when they had the upper hand (Isaiah 10:10-11)," has been quite anticipated by Calvin, when he observes that Moses "leaves the decision to the unbelievers, not as if they would speak the truth, but because he knew that they must be convinced by experience." As a confirmation of this, Luther and others refer not only to the testimony of Balaam (Numbers 23 and 24), but also to the Egyptians (Exodus 14:25) and Philistines (1-Samuel 5:7.), to which we may add Joshua 2:9-10.

Being judges - Who by their dear bought experience have been forced to acknowledge that our God was far stronger than they and their false gods together.