John Calvin Commentary Exodus 12:3

"Speak ye unto all the congregation of Israel, saying, In the tenth [day] of this month they shall take to them every man a lamb, according to their fathers` houses, a lamb for a household:" — Exodus 12:3 (ASV)

Speak ye unto all. A question is asked on this passage: why, when one Lamb alone was offered in sacrifice for the reconciliation of the Church, and God was propitiated by the blood of one Christ alone, should He have commanded a lamb to be slain in every house, as if there were to be a special sacrifice for each one apart? The reply is easy. Although all were protected from destruction by the same blood, and the general rite united them all in fellowship in the same expiation, it was still not unreasonable that, by that special application, so to speak, God would have every family separately reminded, so that it would feel the grace more particularly conferred on itself.

Thus, nowadays we all have the same baptism, by which we are ingrafted in common into the body of Christ; yet His baptism is conferred on every individual, so that they may more surely acknowledge that they are partakers in the adoption, and therefore members of the Church.

God, then, in commanding them to slay a lamb in every house, did not wish to draw away the people to different grounds of hope, but only to show them in a familiar way that all houses were under obligation to Him, and that not only should the salvation of the whole people be confessed to come from Him, but His singular blessing should also shine forth in every family.

The reason for His desiring the neighbors to be added if the number of people in one house were not sufficient to eat the Passover was so that nothing might be left of it. This, among other reasons, appears to have been the chief reason why the whole lamb was to be consumed: namely, so that they would not mix this sacred feast with their daily food, and also so that its dignity would not be diminished by appearing in the form of tainted meat. Perhaps, too, God provided this so that no superstition would creep in from the preservation of the remnants, and therefore commanded the very bones to be burned.