I heard a good vort at shiur yesterday which gave me a lot of chizuk, especially in regard to our struggles.
The first Mitzvah that was given to Klal Yisroel is in this week's sidrah is the Mitzvah of Rosh Chodesh. However, the first Mitzvah that involved an 'action' is the Mitzvah of Korban Pesach, also in this sidrah.
What is the importance of this Mitzvah? Part of the commandment was to put the blood on the doorposts so that the Yidden would not be affected by Makkas Bechoros. Why should the Yidden have been more at risk from this Makkah as opposed to all the others which did not affect them at all? Also, why should blood on the doorposts help?
In the Hagadah we read that Makkas Bechoros was carried out by Hashem Himself and not through a Malach, Saraf or messenger. We can imply that the other Makkos were in fact carried out by Malachim and other heavenly messengers. The reason that the Yidden were not affected by the first nine Makkos is because these heavenly beings have no power over the Yidden, as the Yidden are greater creatures (even when they are on the 49th level of Tumah). To give an example, it is as if the Malachim etc. are pouring dirty water down from the ninth floor of a tower building. The Yidden are on the tenth floor and therefore are not in the 'firing line' and cannot be harmed. However, Makkos Bechoros was carried out by Hashem. He is a much higher source and therefore the Yidden were at risk as well.
The only way that the Yidden could be protected was by becoming part of Hashem's camp. Putting the blood on the doorposts signified that this home was a Mikdash Me'at and part of Hashem's territory. It is a similar concept to an embassy building. For example, the Israeli embassy in England is considered as Israeli territory despite the fact that it is situated in London. Therefore, in Mitzrayim, the Jewish homes were not considered as being within Mitzrayim and were Passed Over. Hence the name of the Yomtov is 'Passover', as this was the key element.
This is the fundamental of Mezuza. By putting a Mezuzah on our door we are declaring that our home is no longer in the territory of the goyim of England, America, etc. but in fact it is the territory of Hashem. Our homes become a Mikdash, totally separate from the schmutz around us.
If we bring the schmutz of the street into our homes, we are totally contradicting the message of the Mezuza. If we do this, our homes become the same as those of the goyim and may not be 'Passed Over'.