After Paroah sent the people (ha-umm) away... (Shemos 13:17)
At first glance, the above verse is talking about the Jewish people. But, as the Ohr HaChaim points out, when the Torah refers to "the people," it means the "Mixed Multitude"--the Erev Rav-- that left with the Jewish people.
Who was the Erev Rav? According to the Ohr HaChaim, they were spies sent along by Paroah to sow seeds of dissension among the ranks of Jews. They were the rabble-rousers, and, though, they may not have been successful at making the Jewish people return to Egypt (which, they came close to doing), they were the cause of Moshe's failure to enter Eretz Yisroel, and the delaying of the Final Redemption. To make a long, painful story short, they represent one of the biggest stumbling blocks in the path of the Jewish nation.
That's right, "represent"--present tense.
This is because the Erev Rav did not die off in the desert, at least not spiritually. "Erev Rav" is a concept, and is a title that can be given to any Jew that tries to dissuade other Jews from belief in Sinaitic Torah, and the Final Redemption. That's what the Erev Rav did in the desert, and that is what the Erev Rav has done in every generation.
The Jewish people have had many enemies from within-- we are a nation with as many splinter groups over the millennia as there have been peoples. (Well, at least it seems that way.) However, what makes the Erev Rav stand out is their motto:
"These are your gods, O Israel, which brought you up out of Egypt!" (Shemos 32:4)
Right, and the next thing you'll want me to do is buy some swamp land in Florida!
Who did the Erev Rav expect to convince with that lie? The answer: Anyone looking for a way out of Torah and mitzvos. When a person wants to turn his back on Torah, all he has to do is call the Erev Rav. It is the Erev Rav that will provide the means and justification--in the name of truth. They'll create whole new branches of Judaism, and then claim that theirs is the true way to serve the Jewish G-d. That's what the Erev Rav of the Jewish people have always done.
For, it is one thing to become secular and leave behind Torah-Judaism (especially out of ignorance). However, it is another thing altogether to turn against Torah and those who uphold Torah, and literally go to war against them. That's what the Erev Rav was sent by Paroah to accomplish, and that's what they declared to do when they entered the religious arena and challenged the Torah's notion of service of G-d.
This is why the verse says that Paroah sent them, and not G-d, as it should have. After all, it was G-d who coerced Paroah into sending out the Jewish people through the Ten Plagues. However, it was not G-d who sent out the Erev Rav--that was Paroah's doing, the posuk is telling us.
The only question is, does the Erev Rav know that they are indeed the Erev Rav of that generation? Probably not--they usually become that way as a result of faulty education, poor decisions, and often, a desire to adapt to the host culture more than the Torah permits.
However, we have to be aware of their presence and the potential damage they can cause in every generation. After all, they are Jewish; they are one of us. But, we part roads with that part of the Jewish people the moment they "attack" Torah Judaism and its adherents, no matter in what name they declare their war.
RABBI WINSTON'S WEEKLY PARSHA PAGE: https://www.neveh.org/winston/parsha59/bshalach.html