As an existentialist play, No Exit necessarily embodies the Sartrean adage "existence precedes essence." As humans, we first exist, with no preconceived expectations, purpose, or ideals to which we must live up. We then define our essence (i.e., who we are) through choice and action. The self is re-created every moment by a conscious choice, and only action dictates our essence and beliefs.
Garcin has a worse case of bad faith at the end of No Exit than when the play began.