A guest post by JS:
Over in the comments on this post, Bray is relating the common idea that the reason the Passover sacrifice is a sheep, is that the sheep was revered in Egypt. Thus, when the Israelites sacrificed the sheep not only was it a "judgment against the gods of Egypt," but it was a gigantic slap in the face to the Egyptians who were powerless to stop the Jews.
However, I can't find any textual support for this idea whatsoever. Yes, the line about "I will execute judgment against the gods of Egypt" is said in the same breath as the Passover sacrifice, but they're not connected - instead the line is related to the plague of the first born. Furthermore, the idea that because the Egyptians found shepherds to be an abomination (Gen 46:34) similarly has nothing to do with this. If it did, presumably they would have prevented the Jews from owning sheep after the Jews were enslaved.
Rather, I think the reason a sheep was chosen is the exodus is a rebirth of the Jewish people and an essential part of that process is going back to the start of the Jewish people. Avraham is told that his descendants will be enslaved and then freed. Almost immediately afterward, he is given the Jewish people's first commandment to circumcise himself. Only when Avraham is circumcised does he merit the birth of Yitzchak. It is through Yitzchak that the defining moment of the Jewish people arises: the akeida. At the akeida, Yitzchak repeatedly asks "where is the sheep intended for slaughter?" Finally, at the last second, Yitzchak is spared and a ram (part of the sheep family) is slaughtered instead.
Thus, when the prophesy related to Avraham is about to come true, when the Jews are about to leave Egypt, we see a return to this theme. Once again, the act of circumcision is intimately tied into the sacrifice of a sheep. Once again, the people are commanded to circumcise themselves and once again they are told to sacrifice a sheep (this time for the Passover offering). In fact, Hashem repeats several times that one who is uncircumcised may not eat from the Passover offering.
As further proof: The sheep for the Passover offering must be male - just like the ram that replaced Yitzchak.
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