The atonement is made through the blood of Christ.
Lev 17:11 For the life of the flesh is in the blood: and I have given it to you upon the altar to make an atonement for your souls: for it is the blood that maketh an atonement for the soul.
The blood had to be applied to the altar before there could be atonement for our sins. This is why Paul said that if Jesus is not risen, then we are still in our sins.
1 Cor 15:17 And if Christ be not raised, your faith is vain; ye are yet in your sins.
It was not enough for Jesus to pour out his blood on the cross. He had to rise from the dead, ascend to the Father, and offer his blood on the mercy seat before there was atonement for our sins. This is why Paul said that if Jesus is not risen, we are still in our sins.
Heb 9:12 Neither by the blood of goats and calves, but by his own blood he entered in once into the holy place, having obtained eternal redemption for us.