John Calvin Commentary Exodus 9:29

"And Moses said unto him, As soon as I am gone out of the city, I will spread abroad my hands unto Jehovah; the thunders shall cease, neither shall there be any more hail; that thou mayest know that the earth is Jehovah`s." — Exodus 9:29 (ASV)

And Moses said. In this answer, Moses indirectly hints that he is leaving Pharaoh's presence to pray to God properly and purely, because Pharaoh's unbelief would, in a way, pollute the sacrifices. For, as Moses had already shown that true worship could not be offered by the people unless they were away from Egypt, he now seeks to be alone for prayer. Thus, by this change of place, he indicates that the place where Pharaoh dwells is unholy.

We have already said that Moses promises nothing from mere rash impulse. Instead, taught either by the Spirit's inspiration or by certain revelation, he declares with prophetic authority what God is about to do. Moreover, it is not without reason that Moses exhorts Pharaoh to learn from the remission of the punishment that the God of Israel is also the Lord of Egypt.

The word 'earth' in this context seems to be limited to Egypt, although I do not deny that it can be properly understood as the whole world. Whichever interpretation you prefer, Moses rightly concludes that God's glory and dominion are fully manifested, not only when He appears as an avenger inflicting punishment, but also when He shows it in the opposite way, with all the elements subject to His mercy.

Besides, His power is even more clearly revealed when He Himself heals the wounds He has inflicted. Therefore, in Isaiah 41:23 and Isaiah 45:7, to prove His divinity, He joins these two aspects together, namely, that it is His prerogative and attribute both to do good, or to do evil.