The line that best supports the theme of underlying guilt is:
"So sensible were the audience of some unwonted attribute in their minister that they longed for a breath of wind to blow aside the veil, almost believing that a stranger’s visage would be discovered, though the form, gesture and voice were those of Mr. Hooper."
This line indicates that the audience perceives something strange or unsettling about Mr. Hooper, suggesting that there is a hidden guilt or secret that lies beneath the surface, represented by the veil.