"For I desired mercy, and not sacrifice; and the knowledge of God more than burnt offerings." -- Hosea 6:6
Even in the Old Testament, God instructs us that the offerings of men are ultimately useless, and that the knowledge of God comes by His mercy alone.
What do the words "Lamb slain from the foundation of the world" mean to you, if not that Christ became a sacrifice for our sins, as God ordained according to His decree at creation?
There is no decree. The Lamb was the sacrifice and the Lamb, the Son of the Holy Trinity was from before the foundation of the world, but God the Father did not demand the sacrifice.