Lambing: A shepherd tells of how they get an orphaned lamb to be adopted. Two ewes gave birth, one ewe died giving birth while the other gave birth to a stillborn lamb. The ewe that survived would not accept the surving orphaned lamb. The shepherds would take the wool off the dead lamb and cover the orphaned lamb with that wool. The surviving ewe would smell the wool and accept the orphaned lamb as her own. Even after she has realized it is not really hers.
So it is with God, covered in the blood of the Lamb of God, the Father does not see us, he sees His Son, Jesus, and by coming in contact with or being covered with His blood, we are forgiven and adopted into the Family of God.