Albert Barnes Commentary Psalms 78:20

"Behold, he smote the rock, so that waters gushed out, And streams overflowed; Can he give bread also? Will he provide flesh for his people?" — Psalms 78:20 (ASV)

Behold, he smote the rock... - See the notes at Psalms 78:15. The smiting of the rock the first time occurred before the complaining about the food. The fact that the rock had been smitten could not be doubted. They had thus had abundant evidence that God was able to do that, and to furnish water for them in the desert. It was unreasonable, therefore, to doubt whether he could provide food for them—for this in itself was no more difficult than to furnish water.

Yet they are represented as affirming that this was far more difficult, and that, although it was admitted that God had provided water, yet that to provide food was wholly beyond his power. Their special sin, therefore, was, that they doubted the power of God in one case, when, in another, equally difficult, they had had abundant proof of it. The spirit of complaining had not been put down by one surprising and undoubted miracle performed in their behalf—a miracle which proved that God had all the power necessary to meet their needs.

Can he give bread also? - Does the ability to cause water to flow from a rock prove that there is also ability to produce bread when necessary? They doubted it, and thus complained against God.

Can he provide flesh for his people? - They supposed that this required greater power than the providing of water, or even of bread, and that if it were admitted that God could furnish the two former, it would by no means follow that he could provide the latter. It was this, as the next verse shows, which was the immediate occasion of the special anger of the Lord.