Left with Stephen, Britomart erupts into tears. She laments that the injustice of a woman's lot is that, after years of work, the father steals the children's affection. She joins the party in the drawing room; "Onward Christian Soldier" is heard on the concertina when she opens the door. Charles sullenly remains in the library.
Major Barbara is commonly grouped among Shaw's "discussion plays"—that is, plays that largely driven by discussion and oratory rather than more conventional forms of dramatic action. Similarly is Major Barbara also at times described as a play of ideas. Its discussions begin in earnest from here, with the meeting of the family in the parlor and its first encounter with Undershaft's gospel.
Undershaft makes his entrance into the household hesitantly, confessing his fear of appearing the "callous father" in playing the "discreet stranger." An indiscretion, however, is his first callous blunder, Undershaft failing to recognize his son. This joke on his protracted estrangement from the household marks his indifference to traditions of familial legacy. As he will later explain, the tradition of the Undershaft firm necessarily trumps that of the household: the identity of his son by kin is of little relevance to him.
Almost immediately after this blunder, however, the charismatic Undershaft establishes a rapport with his estranged family, the play recuperating a father whose children might consider callous into an engaging guest. The major point of contention that emerges here is Undershaft's competing system of morality, what Britomart describes in the previous scene as his "religion of wrongness." Note the reactions of his interlocutors. The heady Stephen bristles at even the suggestion of moral relativism; Lomax hems and haws in discomfort; Barbara remains unperturbed, certain she can save her father's soul. Only the Cusins assents to Undershaft's argument, betraying a fascination with his thought and foreshadowing his conversion to his gospel in the subsequent act.
Specifically, Undershaft poses his faith as a rival to Barbara's Salvationism, though the two are not without similarities. As he jests, the motto of the Salvation Army—"Blood and Fire"—could be his own. Undershaft begins to preach his gospel seductively. Here we only learn that it requires the war and destruction that he underwrites, war becoming more "fascinating" as it becomes more destructive. The destruction Undershaft wreaks seems to promise some form of redemption. Again, "My sort of blood cleanses: my sort of fire purifies" intones Undershaft enigmatically. Bemused by this preaching, Barbara makes a Devil's pact with her father that sets the ensuing action in motion. Each will engage in a campaign for the other's soul.
At this moment, an exasperated Britomart futilely strikes out against this decidedly unpleasant conversation. She does so in the name of propriety. If the family insists on discussing religion, she will call for family prayers. Her protest falls flat, however, the family abandoning her in disbelief for a service in the adjoining room. Britomart bursts into frustrated and cruelly comic tears, lamenting her children's betrayal. Thus Britomart already appears as the abandoned mother, excluded from and irrelevant to the moral struggles afoot between the other members of the cast—struggles that Undershaft, as the play's "great man," has introduced into the household.