“… The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.” Mk 1:15
Responsibility has nothing to do with ability. The words are not even spelled alike (ibility – ability). Man is responsible for his sin but has not the ability to change what he is … a sinner. Nor can he “uncommit” the sin he has committed.
Responsibility in the context of our fallen nature suggests blame, or to be at fault, or guilty before God. We must, and do, bare the blame for our sin. Otherwise God would not be just in legal condemnation, especially in the death of Christ. If we are not responsible then we are a victim of God and the death of Christ would be essentially to justify God rather than the sinner and correct a mistake He has made.
Every sinner that comes before God seeking mercy and forgiveness confesses and owns his responsibility and his inability to render to God payment for it. David, by the inspiration of God makes this very clear in Psalms 51:4 “Against Thee, Thee only, have I sinned, and done this evil in Thy sight: that Thou mightest be justified when Thou speakest, and be clear when Thou judgest”.
The Lord Jesus Christ in His suffering and death assumed the responsibility of His sheep’s sin, taking the blame, being made sin for them as their Substitute. That is one thing. Furthermore, He was, and is, able in the accomplishment of the same to satisfy the just demands of God as He has commanded. We are responsible but have nought to pay. Thank God Christ did … HIS PRECIOUS BLOOD! “Wherefore He is able also to save them to the uttermost that come unto God by him, seeing he ever liveth to make intercession for them” Heb 7:25.
Not only are we responsible for our sin, we are responsible to repent to God and believe on the Lord Jesus Christ. We can do neither. We have not the ability. We are dead in our sin. We do the sinning … God in Christ does the saving. By His assuming the first, He accomplished the second. “For if, when we were enemies, we were reconciled to God by the death of His Son, much more, being reconciled, we shall be saved by His life” Romans 5:10.