What subject was the ministers sermon when he first had the veil on?

Explanation:

he subject of Mr. Hooper's sermon is secret sin—that sin which is hidden from others as well as from oneself. This would seem to be a condition of which Mr. Hooper has direct personal experience: his unexplained decision to wear a black veil over his face every single hour of the day would appear to be related to some secret sin that the minister has himself committed at some point.

Exactly what that sin was, we don't know. But whatever it was, it seems to have had a major impact on Mr. Hooper's life—so much so that it has led him to alienate himself from his congregation by wearing a black veil.

That veil becomes a symbol of how all members of the minister's congregation, as well as Mr. Hooper himself, hide away their sins instead of openly confronting them. The eponymous minster's black veil is unnerving to Mr. Hooper's parishioners, not just because of the grotesque appearance it gives to his face, but also because it acts as a constant reminder of their own hidden sins.