yes it is legal under Title VII of the Civil Rights Act
(2) it shall not be an unlawful employment practice for a
school, college, university, or other educational institution or
institution of learning to hire and employ employees of a particular
religion if such school, college, university, or other educational
institution or institution of learning is, in whole or in substantial
part, owned, supported, controlled, or managed by a particular religion or
by a particular religious corporation, association, or society, or if the
curriculum of such school, college, university, or other educational
institution or institution of learning is directed toward the propagation
of a particular religion.
http://www.eeoc.gov/policy/vii.html