An atheist group is blasting Birmingham Police Chief A.C. Roper.  The Wisconsin-based Freedom from Religion Foundation complained that Roper is mixing his law enforcement duties, with his work as a Christian minister.  The group cited Roper's practice of offering Christian prayers at police gatherings.    Roper also works with a group that promotes prayer as a crime solution.  The foundation charged that Roper is misusing his position to promote Christianity.   A spokesman for Roper said he had no immediate comment.