A January 1970 Associated Press article noted how the producers of this film were the first to film in St. Peter's Basilica, "the second biggest church in Christendom, which the Vatican has allowed to be filmed from top to bottom for the first time in history. The basilica is 435 feet high, 694 feet long, 451 feet wide." The film crew shot "through the night for six weeks under the famous cupola built by Michaelangelo, after the bronze main doors are shut at 6 p.m....Cameras will travel where no tourist is allowed: in small streets tunneled under the basilica and inside the original model of architects Bernini and San Gallo, which is big enough to walk around in." The article explained why executive producer Stanley Abrams and writer-director Harry Rasky succeeded in getting the Vatican to allow filming when earlier productions, such as The Shoes of the Fisherman," could not: "Abrams said it was their desire to show the basilica's conception, growth and history in human terms that sold the Vatican on the project. Vatican authorities always have been reticent to allow traditional acting on its territory, so Abrams settled for 'portrayal.' He stressed that the actors would not act, wear costume or makeup 'which would detract from the story.'"
