Barbara reappears with her father and introduces him as a Secularist. Startled, Undershaft corrects her and says that being a millionaire is his faith. When Shirley declares that he is proud to not be one, Undershaft replies gravely that poverty is nothing of which he should be proud. Barbara asks Shirley to lend a hand in the shelter. He notes bitterly that he must as he is in their debt; Barbara retorts that he must "for the love of them."

Under Undershaft's watch, Barbara continues her assault on Bill. He must join them and "brave manhood on earth and eternal glory in heaven." Just as Bill is about to break down, a drum is heard in the shelter, and Cusins enters. Bill resolves to go to Canningtown and give Mog a good beating and departs.

Barbara returns to her work, leaving Cusins to explain the shelter to her father. Presenting himself as a "collector of religions," Cusins asks into Undershaft's faith. Undershaft believes that two things are necessary to salvation: Money and Gunpowder. Love, truth, mercy, honor, justice, and onward are the graces of a "rich, strong, and safe life." Cusins warns that Barbara cannot endure his dogma.

Undershaft replies that Barbara will soon discover that Cusins himself carries a hollow drum in his own beliefs. Cusins protests his faith and the Army makes men and women out of wasters and worms. It has revealed the "true worship of Dionysus" to the Greek scholar, sending him into the street "drumming dithyrambs." Cusins plays a flourish on the drum. From his own translation of Euripides's The Bacchae, he states that he who knows that "to live is happy" has "found his heaven." Before the Eternal, "Barbara" or "Loveliness" will be loved forever. Undershaft asks if Cusins considers himself a good match for his daughter. With polite obstinacy, Cusins responds that while he fears marriage intensely, he always gets what he feels he must have. He will not stick to the conversion of the Army to Dionysus, as the pathfinder of salvation matters little to him. Undershaft and Cusins shake hands.

Undershaft then talk about how the two of them must win Barbara. Both know that Barbara's power comes not from any doctrine but her own "inspiration." Undershaft intends to give her the business so she can preach his gospel. Cusins remarks that he is mad—a "Father Colossus—Mammoth Millionaire." Reeling with excitement, Undershaft declares Cusins and Barbara mad as well. The millionaire, poet, and savior have no truck with the common mob.

When Cusins protests that he and his fiancée love the people, Undershaft scoffs at their romanticism. Let the poor pretend their poverty is a blessing. Only people who stand above them such as they can help their children rise to their level. Cusins is certain that Undershaft cannot win Barbara. Undershaft replies that he should not underestimate the power of his wealth; thus he will buy the Army. Indeed, the Army serves his interests, keeping the workers sober, domesticated, focused on heaven rather than unions and socialism, and onward. Cusins is overwhelmed.