Also posted on BringMashiach.com
We'll begin with the introduction of Sefer Ner LeRagli by Rabbi Yaakov Yisrael Lugasi (Yerushalayim 5765)
In his introduction, he quotes the Or HaChaim Hakadosh on the Torah (Shemot 3:8). Below is a summary.
When our ancestors were in Egypt, Hashem did not take them out instantly, rather He waited throughout the 10 plagues - a period of 12 months. Furthermore, Hashem waited until the final moment - until it became required for us to leave immediately, in a big rush. Why not? The "ketz" did not come yet. There was a reason to stay there until the final moment.
The main purpose of the Exile is to clarify the "sparks" of holiness that were stuck within the 50 levels of Tumah (spiritual defilement). If they would have left any earlier, their clarification would not have been complete.
Instead, Hashem waited until the final moments, in order for them to complete their descent into the 49th level of Tumah, clarify it, and leave Egypt immediately.
The 50 levels of Tumah are the antithesis of the 50 levels of Bina - understanding (and hence holiness). As all things, they are in direct relation to each other, on completely opposite extremes. Therefore, the levels reached on one side are in direct correlation to the other. And, in order to attain the high level of spirituality on the one side, one must first descend to the lower levels and remove all the holiness trapped within. Therefore, the Jews were required to descend into the 49th level of Tumah, clarify the holiness therein and then deserve the polar opposite 49th level of Bina - receiving the Torah exactly 49 days after they left Egypt.
They were required however, to leave Egypt in great haste. For had they left a moment too late, they would have descended to the 50th level of Tumah and would become stuck there forever. The reason being, that they had not received the Torah and without it, there would be no possibility to ever escape from the 50th level of Tumah.
Baruch Hashem, we have been given the Torah, and so we find ourselves with the tools to combat this lowly 50th level of Tumah, as we will discuss further, Be"H.
Just as in Egypt, we will need to finish this clarification in order to receive the 50th level of Bina, with the arrival of Mashiach - may we merit it soon.
What is this 50th level all about? Stay tuned.