Is it a sin if it does not hurt anyone?

Sometimes we say "I'm not hurting anyone" because we only look at visible consequences. But sin also harms the soul, cools love for God, and deforms the will. Moreover, there are offenses that do not need an audience to be real. A lie, an impurity, inward contempt, or an omission might not appear in statistics, but they do alter the heart. Closing this question with a quick phrase does not help. If it surfaces, it can be a good occasion to form faith better, ask for counsel, and think more calmly. The Church does not fear honest questions; what impoverishes is refusing to search sincerely.