Mortal sin prevents merit.

Passage

While the soul is in mortal sin nothing can profit it; none of its good works merit an eternal reward, since they do not proceed from God as their first principle, and by Him alone is our virtue real virtue. The soul separated from Him is no longer pleasing in His eyes, because by committing a mortal sin, instead of seeking to please God, it prefers to gratify the devil, the prince of darkness, and so comes to share his blackness. I knew a person to whom Our Lord revealed the result of a mortal sin and who said she thought no one who realized its effects could ever commit it, but would suffer unimaginable torments to avoid it. This vision made her very desirous for all to grasp this truth, therefore I beg you my daughters, to pray fervently to God for sinners, who live in blindness and do deeds of darkness.