Marrying An Atheist Woman
Jews are monotheists, and Christians claim to be monotheists with Taweel, therefore Muslims are allowed to marry their women. The same concession can be extended to other communities who claim to be monotheists. However, atheism (not agnosticism) is as great a violation as blatant polytheism since both are lies. Therefore a Muslim is not supposed to marry an atheist.
Extension and restriction are based on Illah (cause). The cause I mentioned in the answer above. Blatant polytheism and also blatant atheism are not accepted however interpretive polytheism or Taweel of polytheism is tolerated. Concession is given only for Muslim men to marry women of the people of the book, this concession cannot be extended to Muslim women. The principle is, that a concession is not extended beyond its scope. Concession requires to be confined.