Christ appeared once in the end of the age to put away sin by the sacrifice of Himself. That work is finished. It can never be added to nor taken away from. Its value does not change. But the Spirit of God works in us to show us our need of Christ's work. He makes us know that we are sinners, that we are lost, in ourselves. He leads us (perhaps by deep and painful convictions) to the sense that there is no good in us; and we learn that, even when to will to do good is present with us, how to perform that good we know not.
We find not only that we have sinned, but that there is a law of sin in our members. This inborn law wars against the law of our minds and brings us into captivity to the law of sin in our members. But when really humbled about this, and convicted in our own hearts, we remove all pretensions of righteousness in ourselves, and turn to Christ, we find that He has died for this very condition. He has been a sacrifice for sin, as well as for the sins that burdened us. He has been made sin for us, and has put it away for us by the sacrifice of Himself.
This is how we get peace and liberty of heart before God. Sin and sins are put away between us and Him. Christ has made full expiation. Sin no longer exists as between God and us.
When He looks upon the blood of Christ He cannot see sin in the believer, because when Christ shed that blood He put away our sin. Thus we get liberty and power too; because submitting thus to the righteousness of God, having Christ for our righteousness, we are sealed with the Spirit, which gives us power and shows us Christ. In believing in Him, we get peace, strength, and joy, and are able to glorify Him.