John Calvin Commentary Exodus 5:3

"And they said, The God of the Hebrews hath met with us: let us go, we pray thee, three days` journey into the wilderness, and sacrifice unto Jehovah our God, lest he fall upon us with pestilence, or with the sword." — Exodus 5:3 (ASV)

And they said, The God of the Hebrews. Moses and Aaron proceed with their message; neither does the pride of the tyrant decrease or weaken their courage in proclaiming the glory of the One true God, who had specially attached Himself to them.

And, certainly, this is the attribute of faith: to trample upon everything that exalts itself on earth, since the truth of God is superior to all human greatness. Nor could they more effectually refute that profane and impious word, I know not the Lord, than by again asserting that the true God is the Protector of their nation, and that this had been disclosed to them in an open manifestation of Himself.

The threatening, which they added, admonishes Pharaoh that his rebellion would not be unpunished if he kept back the people from the worship of God. For if He would take vengeance on the people who were retained against their will, how could he escape with impunity, who openly entered into contention with God? When, then, they declare that some calamity would befall them unless they obeyed the call of God, they imply that Pharaoh must beware of some greater visitation.