Mishnah Torah, Book of Knowledge, Laws of Repentance, Chapter 3, Halachah 4:
Even though the sounding of the shofar on Rosh HaShanah is a decree, it contains an allusion. It is as if [the shofar’s call] is saying:
Wake up you sleepy ones from your sleep and you who slumber, arise. Inspect your deeds, repent, remember your Creator. Those who forget the truth in the vanities of time and throughout the entire year, devote their energies to vanity and emptiness which will not benefit or save: Look to your souls. Improve your ways and your deeds and let every one of you abandon his evil path and thoughts.
Accordingly, throughout the entire year, a person should always look at himself as equally balanced between merit and sin and the world as equally balanced between merit and sin. If he performs one sin, he tips his balance and that of the entire world to the side of guilt and brings destruction upon himself.
[On the other hand,] 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.