12:21-28 That night, when the first-born were to be destroyed, no Israelite must stir out of doors till called to march out of Egypt. Their safety was owing to the blood of sprinkling. If they put themselves from under the protection of that, it was at their peril. They must stay within, to wait for the salvation of the Lord; it is good to do so. In after-times they should carefully teach their children the meaning of this service. It is good for children to ask about the things of God; they that ask for the way will find it. The keeping of this solemnity every year was, 1. To look backward, that they might remember what great things God had done for them and their fathers. Old mercies, to ourselves, or to our fathers, must not be forgotten, that God may be praised, and our faith in him encouraged. 2. It was designed to look forward, as an earnest of the great sacrifice of the Lamb of God in the fulness of time. Christ our passover was sacrificed for us; his death was our life.

22. hyssop—a small red moss [Hasselquist]; the caper-plant [Royle]. It was used in the sprinkling, being well adapted for such purposes, as it grows in bushes—putting out plenty of suckers from a single root. And it is remarkable that it was ordained in the arrangements of an all-wise Providence that the Roman soldiers should undesignedly, on their part, make use of this symbolical plant to Christ when, as our Passover, He was sacrificed for us [Joh 19:29].

none … shall go out at the door of his house until the morning—This regulation was peculiar to the first celebration, and intended, as some think, to prevent any suspicion attaching to them of being agents in the impending destruction of the Egyptians; there is an allusion to it (Isa 26:20).