 According to Brian De Palma (qv) and Art Linson (qv) in the DVD  documentary, it was Sean Connery (qv)s idea to film the "blood  oath" scene between Ness and Malone in a Catholic church. Originally  it was going to take place on the street (in the same scene that  follows the church scene). Connery felt that a church would be the  only "safe" place in Chicago where the two characters would make  such a commitment to fight Capone.