Reverend Hooper's veil is, of course, the central symbol in the story, and its effects on Hooper's life, the lives of his fiance, Elizabeth, and his congregation are profoundly negative. Indeed, many readers, as well as Hawthorne's critics, have wondered why Hooper, upon realizing that the veil's symbolic meaning is misunderstood, doesn't attempt to make his intent clear.
The symbolic meaning of the veil, despite the fact that Hooper's congregation is too unnerved to understand it, is made explicit in the sermon Hooper gives the day he puts the veil on:
The subject had reference to secret sin, and those sad mysteries which we hide from out nearest and dearest, and would fain conceal form our own consciousness, even forgetting that the Omniscient [that is, God] can detect them.
We know from the narration that everyone who heard this sermon feels as if Hooper has looked into his or her heart--from "the most innocent girl, and the man of hardened breast"--and discovered those secret sins of which the veil is the symbol. One can argue, based on this detail, that the veil's meaning is not a mystery but rather that its meaning is consciously pushed aside.
A question that flows from the veil itself is, "Why choose a veil to symbolize one's secret sins?" The answer is, I think, in the eyes behind the veil. A common refrain in the 17thC., as well as in the 21stC., is that the "eyes are the windows of the soul" or, as Alexander Pope expressed it, the "eyes mirror the soul." By wearing the veil, Hooper is closing down not only one of the most important of our senses but also the sense that we all depend upon to judge how another person is feeling. There's a reason, for example, that police officers, when questioning someone, want to look into that person's eyes. One's voice can be disguised; the look in the eyes cannot. For Reverend Hooper, then, masking the eyes with a veil--the windows of the soul--is a natural emblem to represent everyone's tendency to "mask" his or her soul from others, the place where secret sins reside, protected from disclosure.
Hooper's choice of the veil to mask the eyes is a perfect way to evoke the concept of hidden sins, but one of the enduring questions about this story is whether or not Hooper was entirely effective in his attempt to teach his congregation to take off their own "veils." After all, he essentially ruined his own life, blasted the potential happiness of his fiance, and created a gulf between his congregation and himself in order to carry the symbol to his deathbed. Does the veil successfully evoke the theme of secret sin? Yes and no.