The Rebbe here explains the term "Erev Rav" - a term many Jews, until today, still get wrong. I break it into a few parts, after the last of which you'll understand the connection between the Erev Rav and the Holocaust.
In only five simple words Torah neatly dispenses with the entire issue of the Erev Rav:
וגם ערב רב עלה אתם
A big mix joined them.
And what were their demographics? Here too opinions vary. Holy Rashi believes "they were converts from variegate nations". The Midrash, in a few places, says they were "Kosher Egyptians". The Zohar (191:1) says they were "Egyptian Magicians".
During processions through the desert, or when camped, where did they hang out? Rashbi says "the Clouds of Glory did not embrace them [i.e., they dwelt outside the camp proper], and in procession they followed after the livestock." "The Manna", he says, "did not descend for them; They would get their food from leftovers Jews gave them."
Besides the above inconclusive data, many questions remain on all the above standpoints. It makes no sense Egypt had in their midst that many "Kosher" Egyptians; Or that many "Magicians". And why does Torah remain silent about them; Like, did they or did they not circumcise themselves? Or, how did they fit into familial accounting after the Sinai event, or where there place was in the Jewish encampment's configuration? Did they marry, have children? Did they enter Eretz Yisroel in disregard of the law that Egyptians must not merge with the Jewish community until the 3rd generation? For if they did enter, this could mean the genuine Jews became diluted with 5 or 10 times 600,000 converts - a fact that's hardly credible.
After the Golden Calf affair, when Moshe prayed for the Jewish people, his main argument based itself on Jewish lineage with their forefathers. He said they were descendants of Avrohom, Yitzchok and Yakov. He said not one word about converts that led astray the Jewish people. When Moshe ordered the Levite tribe to kill those who worshipped the Golden Calf, he said to kill "a man his brother", a man his friend and a man his relative". Later, when he praised these Levites, he spoke of killing "a man his own son and a man his own brother". This means those who died were Jews! Nothing was said of converts who died. What our sages did tell us was the Erev Rav bore responsibility for persuading the Jews to stray, and that Moshe brought them out of Egypt on his own accord.
When prophets, later in history, punished the Jews, they reminded the nation of their illustrious forefathers:
."הביטו אל צור חצבתם ... אל אברהם ... ואל שרה"
Certainly the prophets could not have said this had the Jewish nation absorbed 3 million Egyptians, converts or other nationalities. That the Holy Books make refer to the Jewish nation with terms such as
"שבטי ישורון" , "בית יעקב" or "שבטי ישראל",
proves their familial lineage stems from Jewish forefathers. Had the Jewish people been a minority among those who left Egypt, then a few generations later it would have been impossible to lay claim to the pure pedigree of the Jewish forefathers that ran in their blood.
Otherwise, the Jewish people would not have been a nation but some mass of people clinging to a Moses-given religion, tugging about after him in the desert. What purpose could it have served the true children of Yakov to submit to persecution when foreign converts got the same privileges with no hardships to endure and no sorrow to taste? Why did God never once mention anything to Moshe about taking converts out of Egypt - when all God ever spoke about were the children of Israel and His promise to the forefathers?
Can anyone really believe the Erev Rav who dragged the Jews into all the quarrels in the desert were actually real Gentiles; Or that Jews would allow themselves to be so influenced by Gentiles? Or, that Moshe would refuse to send them away (but rather took upon himself they should come along)? Or that Korach and his cohorts, his family and the many Jewish leaders he influenced, God allowed be swallowed by the earth, but Gentile converts God wouldn't touch? Or, had Gentiles actually wanted to convert, that they would then have the audacity to oppose Moshe?
Many more questions remain on the Erev Rav topic had their real origin been from conversion. But the main question would involve the bond God made with the forefathers, as when God told them their descendants will be "a kingdom of priests and a holy nation." If 5 or 10 times that many converts shared the paternal privileges accorded Jews, then the entire covenant made with Avrohom, and the promise "for your children shall be strangers in a land not theirs" (כי גר יהיה זרעך בארץ לא להם ...) would amount to nil!
This is what Christians say in their teachings - that one need not be from Avrohom's lineage in order to be "The Chosen People". It's enough, they say, to be spiritual children of Avrohom. They make this claim so they can thereby take over the special status of being "The Chosen People".