These two lands: the one where Klal Yisroel had dwelt for 210 years, and the land to which they were headed, were the most morally depraved lands in the world. By stressing these two lands – where Klal Yisroel were in the past, and where they were headed – the Torah teaches us that they were worse than all the rest.
The Toras Kohanim says that because Klal Yisroel dwelled there, it caused them to be even more sullied. The Sitra Achara can only attain its sustenance through Kedushah. Due to the sins of Klal Yisroel, they can latch on to the kedushah from Klal Yisroel, and thus give themselves sustenance. The more that Klal Yisroel sins, the more the forces of evil receive. Thus, they are constantly seeking to cause Klal Yisroel to stumble and sin. Hakodosh Baruch Hu Himself, is Mashgiach over Klal Yisroel. It says in Bereishis Rabbah 20 that the yetzer hara puts evil thoughts into the minds of Kayin and his friends (groups of evil people), and lehavdil, Hakodosh Baruch Hu places thoughts of kedushah into the minds of Klal Yisroel. Anything that the yetzer hara can do, to sneak in, to get Klal Yisroel to sin, it certainly does. It says in Tehillim 106:35
“Mingled with the nations, and learned their ways.” This does not only mean that they mingle together and actually see their evil deeds, but even just living among them without seeing their practices has a profound effect on them.
The other nations of the world, sullied with their evil thoughts and actions, the very air in which they reside, as the Rama Mipano says, that this evil becomes part of the air. Chazal tell us that the air of Eretz Yisroel causes one to become smarter – it is because of the quality of the air. The yetzer hara doesn’t have direct access to Klal Yisroel, but rather it comes at them from the side. It gets the other nations of the world to perform many abominable acts against Hakodosh Baruch Hu, and that sullies the air. The ziddin who are in proximity to that air are adversely affected by it. Thus, the posuk is telling us that Klal Yisroel just living there, their being in tumas Mitzrayim, was a great danger for them, and extremely harmful to their ruchniyus.
Thus it says in Vayikra 23:3 “It is a Shabbos for Hashem in all your dwelling places” – Shabbos is a time that the evil spiritual powers are banished, and thus it is Shabbos, “in all your dwelling places” – at all times, even when we are in galus. The air is pure, for all the evil powers are gone, and thus it is appropriate for one to strengthen himself in Torah and tefillah on the holy day of Shabbos, for it is a day on which the air is always tahor.
