A mortal sin is a very bad thing.
The
Catholic Church classifies sins according to their
gravity. For an action to be a mortal sin, three criteria must be satisfied:
- The sin must be grave (ie murder, stealing)
- It must be done with full knowledge
- It must be done with full consent
Full knowledge means that a person knows what he is doing is sinful, thus if a person does not know that skipping mass is a mortal sin, then he does not commit a mortal sin by skipping mass.
Full consent means that a person forced to do something sinful against his will does not commit mortal sin. These do *not* mean that the action itself was not sinful, only that it is not classified as mortal.
The consequence of mortal sin is eternal punishment (hell). Thus, a person who dies in the state of mortal sin *may* go to hell.