What was the subject of Mr. Hooper's sermon on the day he began to wear the veil? How did the veil affect his topic?
During the first sermon that Mr. Hooper gives once he begins to wear the veil, he addresses "secret sin, and those sad mysteries which we hide from our nearest and dearest, and would fain conceal from our own consciousness, even forgetting that the Omniscient can detect them." In other words, Mr. Hooper talks about the fact that every human being has a secret, sinful nature; however, we try to hide that nature from all our fellows -- indeed, we try to forget about it ourselves as well -- and we can even delude ourselves into thinking that God, himself, is unaware of our secret sins. And although Mr. Hooper delivers the sermon in much the same way as he ever has, gently and beseechingly rather than violently and threateningly, his audience considered it to be "the most powerful effort that they had ever heard from their pastor's lips [....] and [...] the hearers quaked." Thus, it must bear the stamp of truth because they feel it to be more persuasive and evocative than any other sermon of his that they have heard.
The veil's symbolism, then, is the subject of this sermon. It is a tangible representation of our desire to hide our true natures from the world. If one fears revealing their secret sinfulness to the world, it is as though they hold up a figurative veil between themselves and everyone else. Therefore, when Mr. Hooper discusses this sinfulness that we insist on trying to hide, he is really explaining what the veil, itself, means.