How could the Rav say that galus Mitzraim was meant to purify our nation if there was such a big danger of assimilation?
That’s exactly it! That’s what purified us. Fighting back against the influences of the outside world of sheker is what makes you great. That’s what purifies you.
They began their stay in Mitzraim by training themselves under the direction of Yosef Ha’tzadik in how to fight back against assimilation. When Pharaoh called in Yosef’s brothers for the purpose of giving them big positions in the government, Yosef warned his brothers. Say, “We are just plain shepherds.” “Ro’ei tzon avodecha, gam anachnu, gam avoseinu ” (Bereishis 47-3).”We are nothing but shepherds. We are not fit for any big position.”
Now, the brothers desired to get good positions. They would welcome any promotion that Pharaoh would give them. But Yosef warned them, “Nothing doing!” And throughout his whole life, Yosef kept on warning his brothers and the whole family, “Do not become assimilated with the people of Mitzraim. Do not let your ideals become assimilated with the ideals of Mitzraim. Don’t mingle with them. Maintain your Hebrew names. Speak your own language. Everything should be according to the way our Avos lived.”
And that struggle, that audacity, to remain separate was a tremendous achievement. That was the success. That was the preparation for the battle against assimilation. And that was what saved them and that’s why they eventually were taken out of Mitzraim. So it was Yosef, who demanded that separation, that aloofness, from the ways of the Mitzrim, who deserves the credit for training the Am Yisroel to be deserving of Yetzias Mitzraim.