Upon completing Help a Brother Out, advance the time to the next day and you’ll find Brother Dorkins at a north side Church in Saint Denis. Once you begin the mission, a kid will take off with the Sister’s crucifix and you’ll need to chase him down quickly if you want that Gold Medal.
This is easy when you know where you’re going so that you don’t waste time asking questions to strangers. Dash down the street and take a left, follow it to the end and then take a right and head up the street there to find the kid. Antagonize the man to let the kid go and he’ll run off leaving the crucifix on the ground.
Unfortunately for Arthur, Mrs Downes has taken to the street corner now and alerts the police when she spots Arthur, quickly dart to the north using the alleys to escape the Law. You’ll now need to make your way back to Sister Calderon without running into anymore of the Law. The only way to access her is by heading through the back of the Church, use the alleys to approach it from the south and then access the courtyard to simply stroll right up to Sister Calderon and complete the mission.