"It is at this point we find Dante clearly distinguishing between Reason and Revelation. The secular order is founded upon Reason, and its task is to lead happiness in this world; the spiritual order is founded upon Revelation, and its task is to lead men to eternal beatitude. "
If you know the answer straight away, keep it to yourself in order to give others a chance to guess. And as always, no Googling.... .
.Commenter Barrett Turner got the answer right, the quote is from Dorothy Sayers's introduction to Dante's Inferno.