Why does it say "Mi He'emin li'Shmu'asenu"?
Rashi: So Nochrim will say to each other - had we heard what we see, it would not be believed.
Radak: Nochrim will say, who would have believed what we heard from the Nevi'im, or those who cited them. We would not have believed what we see now with our eyes.
Malbim: This refers to above. Why did no one tell them a possibility like this until now? Until now, it was considered impossible, for two reasons. (a) The tidings themselves - the miracles that they will hear are far from nature. They did not believe until now that Hashem would do wonders like these. (b) They did not believe that Hashem's arm would be revealed on a nation lowly in their eyes.
What is the meaning of "u'Zero'a Hashem Al Mi Niglasah"?