The sin of presumption is the sin of believing you can commit any sin, die unrepentant of it, and still go to heaven, or alternatively, that you can commit any sin and expect the grace of repentance as if it were your due.
A Catholic can have a moral, though not an infallible, certainty that he or she is in a state of grace and an heir of heaven.
Regarding this...In a sermon I heard a story about St. Joan of Arc. During her trial they tried to trap her by asking her if she was in a state of grace. She gave an excellent answer, she said, "If I am, I pray God keep me there. If I am not, I pray God put me there."