If I am at work and a woman who is immodestly dressed (according to what I consider immodest) comes in to buy something, and I can’t help but look at her (since we’re talking face-to-face) how do I know if I am guilty of the sin of lust? And am I guilty of a mortal or a venial sin? Thank you.
Sin can only be the result of a choice to sin. If you did not will to be tempted, you certainly did not sin. Simply because you feel aroused by responding to her need to have you look at her for the transaction, you did not sin. You have no control over what she chose to wear. When the transaction was completed, THEN you were on your own. IF you continued to watch her leave for the pleasure it gave you, then you could have sinned venially—or even mortally, depending on the degree of your continued interest.
Fr. Vincent Serpa, O.P.