"If he performs one mitzvah, he tips his balance and that of the entire world to the side of merit and brings deliverance and salvation to himself and others. This is implied by [Proverbs 10:25] "A righteous man is the foundation of the world," i.e., he who acted righteously, tipped the balance of the entire world to merit and saved it."
-- Maimonidies laws of Teshuva Chapter 3
