The line that best supports the theme of underlying guilt is:
"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 and discovered their hoarded iniquity of deed or thought."
This line conveys the idea that the veiled presence of Mr. Hooper evokes feelings of guilt and self-awareness in the congregation, suggesting that they are conscious of their own sins or hidden wrongdoings.