The Minister's Back Veil is a parable or allegory because is contains a moral message. When Hooper first puts on the black veil, everyone expects it just to be a prop for his sermon. The subject [of his sermon] had reference to secret sin, and those sad mysteries which we hide from our nearest and dearest. The effect the sermon had on the parishioners was extraordinary.
A subtle power was breathed into his words. Each member of the congregation, the most innocent girl, and the man of hardened breast, felt as if the preacher had crept upon them, behind his awful veil.
However, the people expect him to take off the veil at some point that day. The day ends and Hooper continues to wear the veil. His fiance even says she will not marry him if he continues to wear the veil. Hooper replies, "There is an hour to come,'' said he, "when all of us shall cast aside our veils. Take it not amiss, beloved friend, if I wear this piece of crape till then.'' In other words, he intends to wear the veil until he dies. So she breaks their engagement but he still wears the veil. On his deathbed, he still refuses to take off the veil but says instead, "I look around me, and, lo! on every visage a Black Veil!" In other words, the moral message that Hooper was trying to make is that everyone has some kind of secret sin that they try to hide. That moral message, coupled with a symbol ( the black veil) makes this story a parable.