This week's Torah portion, Bo,

This week's Torah portion, Bo, contains the account of the tenth and final plague which G-d visited on the Egyptians, the only one in which the Jews were required to put a mark of identification upon their homes so that they would not also be afflicted when the firstborn sons were killed. The Jews were commanded to put blood from the Passover sacrifice on their doorposts, also symbolic of the blood of the covenant of circumcision, and warned to remain in their homes until the morning.The Midrash offers an explanation why these precautions were necessary: "Once the Destroyer is given free reign, it cannot distinguish between the righteous and the wicked." This is why a special sign was needed to divert the Angel of Death. But wasn't the Angel of Death allowed free reign during the previous nine plagues? Why weren't measures taken then by the Jewish people to protect themselves?
The answer lies in the fact that the slaying of the firstborn was essentially different from the plagues which preceded it. The first nine plagues brought a limited and specific type of injury and devastation; the Angel of Death was not allowed to indiscriminately destroy in whatever manner it chose. During the tenth plague, however, the Egyptian firstborn died in a multitude of different ways.
Even more fundamental is the fact that the aim of the first nine plagues was to make the Egyptians acknowledge the existence and power of G-d. The final plague was sent solely to punish and to kill.
At this point, the Attribute of Justice complained before G-d and pleaded that the Destroyer be allowed to harm the Jews as well: "How are the Jews so different from the Egyptians? Both nations have served idols, and both nations have sunk into the 49 gates of impurity!" G-d therefore decreed that the Jews identify themselves with a special sign, so that no harm would befall them.
But how could a drop of blood on a doorpost defend the Jews against such a grave accusation? Chasidic philosophy explains that the tenth plague was visited by G-d Himself, and was a demonstration of G-d's overwhelming love for the Jewish people, the love a father has for his children. This is a love irrespective of the son's negative behavior; it transcends even the legitimate claims of the Attribute of Justice.
The blood with which the Jews painted their doorposts was symbolic of the essential connection which exists between G-d and the Jew, a bond which transcends all rationale and human understanding. Just as the command to publicly defy Egyptian sensitivities by slaughtering a lamb, the Egyptian deity, seemed to be irrational, it was precisely this disregard for the natural order and the desire for self-sacrifice which brought about the redemption. It is only when Jews go beyond the boundaries of logic to show their devotion to G-d that He repays in kind.
Today, the Jewish people finds itself in a situation similar to the one faced by the Children of Israel as they were about to leave Egypt. The Final Redemption is right at our door, and all that is required is that we transcend the bonds of rational deliberation and declare ourselves ready. In this merit may we see the coming of Moshiach and the dawn of the Messianic Era.

Adapted from the works of the Lubavitcher Rebbe