http://revach.net/article.php?id=3360

Section: Taryag Mitzvos   Category: Insight
Parshas Bo: Rambam Moreh Nevuchim - Korban Pesach Was A Bold Act To Free The Mind
Bnei Yisroel in Mitzrayim were influenced by the Mitzrim and worshiped their Avodah Zara the sacred lamb.  The Rambam in Moreh Nevuchim (3:46) says that the Korban Pesach was meant to eradicate this belief from Bnei Yisroel before leaving Mitzrayim.  Each household gathered the whole family together to witness the slaughter of this idolized animal.  This kind of rebellion in something that they believed in was very courageous.  Not only that but despite their terrible fear of the consequences, they were commanded to smear it on the outside of their homes to publicize it to the Mitzrim, who were surely outraged.

To those who still had doubts about their actions Hashem taught them a lesson.  Not only will they not be harmed by the vengeful Mitzrim or the supposed G-d for their brazen act, they would even be protected by the smeared blood.  "V'Lo Yitein HaMashchis Lavo El Bateichem".  While the Mitzrim were being slaughtered by the forces of destruction, the homes of Bnei Yisroel were protected by the blood on the doorpost.