st Thomas Aquinas on sin and restoration

Man cannot rise from sin without the help of grace. When the actual sinning has stopped, certain damage has been sustained, so to rise from sin is not simply to stop sinning, but to have that damage repaired. The damage is threefold: a stain, by which we mean an eclipse of the brightness of grace by the tarnish of sin; a disorder of nature, in which man's will is no longer subject to God; and a liability to punishment, since mortal sin deserves to lose God forever. Only God can repair such damage: he alone can shine his light on the soul again and restore the gift of grace-as-disposition; he alone can draw man's will to submit itself again to him; and he alone as plaintiff and judge can remit the liability to eternal punishment. So to rise from sin man needs the help of grace, both as disposition and as inner activation from God. Men cannot recuperate from sin on their own, because the light of grace has to be instilled in them afresh; just as a body cannot recover from death unless its soul is instilled afresh. Even an integrated nature couldn't have restored itself to a state beyond its natural capacity without external help. Human nature is now dissipated by an act of sin, no longer integrated but in disorder; so it cannot even restore itself to its own natural goodness, never mind to a state of rightness beyond its nature. ~ St. Thomas Aquinas, ST II.II.109.7 (ed. McDermott, 310)