John Calvin Commentary Deuteronomy 7:17

"If thou shalt say in thy heart, These nations are more than I; how can I dispossess them?" — Deuteronomy 7:17 (ASV)

If you shall say in your heart. Since it was a matter of great difficulty to destroy such a multitude of people, and despair itself would drive them to madness, so that it would be pointless for the Israelites to cut off all hope of mercy, God anticipates their fear and exhorts them to the strenuous execution of His sentence.

From this we gather some useful instruction: whenever God commands anything that exceeds our power, we must still obey and boldly break through whatever obstacles present themselves to impede us. In all arduous matters, therefore, let this doctrine come to our aid: that whatever is contrary to God’s will may easily be annihilated by His almighty power.

But since terror, when presented to our eyes, immediately so grips all our senses that we lie, as it were, torpid, God reminds the Israelites of the abundant grounds for confidence He had supplied them with. For all the miracles He had performed were so many proofs of His invincible power; and therefore, they should conclude that nothing was to be dreaded, provided God went before them, and that, consequently, being assured of victory, they should not descend to any treaties.