Every person must view himself all year long as though he is half meritorious and half culpable, and also the entire world is half meritorious and half culpable. [Therefore,] if he commits one single sin, he will have tilted himself and the entire world to the side of culpability, and cause them destruction. But if he does one mitzvah, then he will tilt himself and the entire world to the side of merit, and cause himself and them to be saved.
Rambam’s Mishneh Torah, Hilchot Teshuva 3:4, based on Kiddushin 40a-b
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