See the IRS's "Ministers Audit Techniques Guide" for guidelines on classifying an employee as an employee.
First of all they MUST be duly ordained, commissioned, or licensed minister of the church.
The rest of the guidelines include:
-Administer the sacraments of the church (weddings, funerals, baptisms, and communion, etc...)
-Be considered a religious leader by the church
-Conduct religious worship
-Have management responsibilities in the church
They don't have to do all of the 4 guidelines above...but should be doing most of them to be considered a minister by the IRS.