17 As yet exaltest thou thyself against my people, that thou wilt not let them go?

18 Behold, to morrow about this time I will cause it to rain a very grievous hail, such as hath not been in Egypt since the foundation thereof even until now.

19 Send therefore now, and gather thy cattle, and all that thou hast in the field; for upon every man and beast which shall be found in the field, and shall not be brought home, the hail shall come down upon them, and they shall die.

20 He that feared the word of the Lord among the servants of Pharaoh made his servants and his cattle flee into the houses:

21 And he that regarded not the word of the Lord left his servants and his cattle in the field.

God makes a most bold prophecy through His servant, Moses. He says that not only will hail fall upon Egypt, but one “such as hath not been in Egypt since the foundation!” This would be a storm such as had never seen before by the Egyptians, not even since the founding of the nation. No one could make a promise like this except God. No one could possibly anticipate such a rare and momentous event except the all-knowing. And certainly no one could deliver on this promise except the all-powerful. At this point, the curses against Egypt were firmly in the category of natural disasters!

And with this warning God also provided a test. He had already made a division between the Egyptians and the Israelites, now He would also distinguish between the God-fearing and the God-doubting Egyptians. By forewarning them, the people of the land knew that they could spare themselves by staying in shelter. Doing so would show that they believed in the Hebrew God and doubted the priests of Egypt, while on the other hand, those who wished to show support of the local gods and renounce the power of Elohim had only to stand out in the open. To do so would be gambling with their very lives!

Or, at least, would be gambling with the lives of their servants and cattle. Verse 21 suggests that the rich and powerful of the Egyptians did not stand out in the open themselves, but rather they required their servants to continue to work the field and die for them. They stood to lose value and resources only, but to make their own people suffer a terrible demise. Of course, we will soon come to the final plague, and in that one not even the rich and the powerful would have any refuge.