Christ's blood that takes away sin was not shed until He died on the cross THEN sins could be taken away (Heb 9:26-28). Therefore the forgiveness under the OT law was in promise looking to the cross and Christ's shed blood.It also appears that you are negating the fact that God see’s that the Lamb was slain before the foundation of the world and those names were in the Lamb’s book of Life before the world began.
All who dwell on the earth will worship him, whose names have not been written in the Book of Life of the Lamb slain from the foundation of the world. Revelation 13:8
JLB
Rev 13:8 God's plan in redeeming man from His sins was devised and happened in the mind of God before the foundation of the world (1 Per 1:19-20) and carried out at the cross when Christ came to earth to do the will of God..