Yet for polemical value the author or source name can be of much value, but i understand your concern is only copyright.
Bible verses and the Constitution are in the public domain.
Indeed, and easily recognizable for what they, unlike many other public domain sources, and i would assume that in such a case you would want to know the source in order to make sure. Plus it is ethical. I cannot see quoting swaths from the Westminster Confession or Catholic Encyclopedia without attribution.
From the Religion Moderator profile page:
http://www.freerepublic.com/~religionmoderator/
“When quoting a source, e.g. a website, article or book be sure to include sufficient source information for the moderators to enforce copyright restrictions.”
Every quote from a website, article or book should be attributed so we can check for copyright violations.
With the exceptions posted earlier, obvious public domain quotes.