My sacrifice is the Passover. It is the only sacrifice within the terms of the Old Covenant, yet we keep it under the New Covenant! Why?
When did sacrifice began? It began long before the Old Covenant—in fact, it began at the beginning. The earliest record of a sacrifice is Cain's and Abel's sacrifices. Where did they learn of it? They undoubtedly were taught it by Adam and Eve, who learned it from God. They did not just dream up this idea of offering the Deity an animal in sacrifice. The implication is that they were commanded to do it by God (Genesis 3:21 may have been the occasion of the first animal sacrifice). Cain and Abel knew what God required.
We do not offer sacrifices today, due to Christ's sacrifice once for all (Hebrews 9:12-14), but we do offer the Passover. The other sacrifices that we do not offer are never mentioned in the Old Covenant. Yet here is the Passover, which is mentioned in the Old Covenant, and we are commanded to keep it (I Corinthians 11:23-26).
The Bible is an expanding revelation. Just because something appears in the Old Covenant does not mean that it is obsolete and done away. We understand that some aspects of it are no longer required of us, but we still keep Passover in its New Covenant application.
John W. Ritenbaugh
The Covenants, Grace, and Law (Part 17)