The First Vision makes it pretty plain (JS - History 1.19). See also D&C 1:30. McConkie's Mormon Doctrine offers an excellent summary of the Church's position, s.v. "Church of the Devil."
Your response actually stands in line with the reaction I get from all my Mormon friends. It's like they've never heard this notion that if theirs is the only true church on earth, the others are by definition false. I don't know if it's the new younger generation of Mormons or something, but my peers seem very unfamiliar with this church doctrine and its implications.
*edited for typo
Mormon Doctrine was not published by the Church. If you want church doctrine, you need to find it in church publications at the very least.
This is how it is taught to us: http://www.lds.org/ensign/1988/01/warring-against-the-saints-of-god?lang=eng
Edited by calmoriah, 27 April 2012 - 11:11 PM.