The Sages relate that when the Israelites were in Egypt they were completely steeped in idol worship, like their Egyptian neighbors. The first mitzva they were commanded was to take a lamb and slaughter it. This act was a clear departure from their path of idol worship, and is what enabled the redemption.

Likewise, you find concerning Israel that when they were in Egypt, they would worship idols and would not forsake them, as it is stated: “No man cast out the repugnancies of their eyes, and they did not forsake the idols of Egypt” (Ezekiel 20:8). The Holy One, blessed be He, said to Moses: As long as the Israelites worship the gods of Egypt, they will not be redeemed. Go and tell them that they should forsake their evil actions and repudiate idol worship. That is the meaning of what is written: “Select [mishkhu] and take for yourselves” (Exodus 12:21); in other words, withdraw [mishkhu] your hands from idol worship, and take sheep [which were considered sacred by the Egyptians] for yourselves, and slaughter the gods of Egypt, and perform the rite of the paschal offering [pesah]; as through this, the Holy One, blessed be He, will pass over [pose’ah] you during the plague of the firstborn. (Shemot Rabba 16)