Take the new Silver line (a new Bus Rapid Transit service that runs between Temple Place at Downtown Crossing and Dudley Station in Roxbury) and get off at "Union Park Street" stop. The Church is 15 Union Park Street just seconds from the stop.
You can connect to the new Silver line from:
Red Line: Get off at Downtown Crossing at the corner of Summer St. and Washington St. Take Washington St. towards Chinatown (along Macy’s store) for one block. On Temple Place turn right. The bus stop is on your right (almost at the corner of Temple Pl. and Tremont St.).
Green Line: Get off at Boylston Station. The Silver line bus stop is on Tremont street, right across "Lowe's Theaters"
Orange Line: Get off at New England Medical Center.
The Silver line bus stop is in from of the main entrance to the New England Medical Center
(Make sure you get a transfer ticket so you do not pay for the Silver line)

Get off at Copley Square on the Green Line or Back Bay Station on the Orange Line. The Church is a 10min walk in beautiful South End. Take Dartmouth Street towards Tremont Street (not towards Newbury street or Charles River). On Tremont street turn left. On the second traffic lights Union Park street is on your right hand you. Turn right on Union Park street and cross Swamut Ave.
The Church is on your right hand side. (Washington Street is on the next crossing).