This principle, known as the "ministerial exception,” isn’t explicitly found in the First Amendment, which in part bars laws prohibiting "the free exercise” of religion. Rather, it was adopted by a federal appeals court in a 1972 case and has since been endorsed by several others, although courts disagree on its scope.
Earlier this year, the U.S. Supreme Court announced it will hear a case dealing with the exception. The dispute stems from a grievance filed by Cheryl Perich, a Michigan woman who says she lost her job at the Hosanna-Tabor Evangelical Lutheran Church and School after being diagnosed with narcolepsy – even though a doctor had given her medication that controlled the condition.
Title VII of the act exempts any "religious corporation, association, educational institution, or society” when it comes to hiring individuals carrying out religious work. The ministerial exception, by contrast, is a court-crafted rule that has been interpreted to cover any form of discrimination, religious or otherwise.
http://www.au.org/media/church-and-state/archives/2011/10/hosanna-in-the-highes.htmlIt's a long article but worth the read considering the implications of a SCOTUS ruling on this. - TS