/lit/ - Literature

Apparently he wrote this in a letter in response to a book about religion. Freud thought humans are animals who have been regulated by civilization, not souls which were created by a God, so he didn't think there was any intrinsic, natural goodness to people, so whatever "goodness" there may be is instituted by civilization on otherwise base animals.
He also wrote that in 1918, and he was immensely disillusioned and butthurt about WWI, so take the cynicism with a grain of salt; I doubt he'd say anything that strong ("trash") at any other time in his career.