The blood of Christ applied to the sinner is atonement. How is it applied? It is applied only by spiritual union of Christ and the believer. Only after Christ is in me does God see me as if it was me who had lived His righteous life and died His atoning death. Only after He is in me does what He did count as having been in my place. Only when the Christ of the cross is in me does His death propitiate God in my case. Until then, I remain under wrath and my sins have not been atoned for.