exclusion

I found a post about GodBlogCon 05 and it sounded interesting, so I went to their site. But then on the site I read this and was completely turned off.

Because GodBlogCon is a Christian conference, participation will be also be limited to those who affirm the historical and Biblical Christian faith.

I guess I can't go because I'm not sure what that even means. And why do they feel the need to exclude on that basis anyway - not up to a challenge?

.. many Christian institutions have a system by which you find out whether you're in or out. Sometimes it's rules; sometimes it's a certain belief system. - Richard Foster, The Making of a Christian interview at Christianity Today

Why do so many Christian institutions feel the need to exclude?