Our best duties are as so many splendid sins. Before you can know you are at peace with God, you must not only be made sick of your original and actual sin, but you must be sick of your righteousness, of all your duties and performances. There must be a deep conviction before you can be brought out of your self-righteousness; it is the last idol taken out of your heart.
...as Adam and Eve hid themselves among the trees of the garden, and sewed fig leaves together to cover their nakedness, so the poor sinner, when awakened, flies to his duties and to his performances, to hide himself from God, and goes to patch up a righteousness of his own. Says he, "I will be mighty good now — I will reform — I will do all I can; and then certainly Jesus Christ will have mercy on me."
But before you can know you are at peace with God you must be brought to see that God may damn you for the best prayer you ever put up; you must be brought to see that all your duties — all your righteousness ... are so far from recommending you to God, are so far from being any motive and inducement to God to have mercy on your poor soul, that he will see them to be filthy rags.
Snippets taken from his Sermon, "The Method of Grace"