Known as one of Shaw's "discussion plays," Major Barbara is primarily structured through a series of conversations on morality, religion, and social engineering. The primary topic of discussion is what Shaw identifies in the preface as the "Gospel of Saint Andrew Undershaft," that is, the gospel that would promise society's redemption.
Undershaft's gospel is organized around the apotheosis of the millionaire and, more specifically, the military industrialist. As the characters will come to realize, the world is not in God's power but in the power of the military industrialist. With money and gunpowder, Undershaft participates in the power that reigns over Europe, the power that determines the course of society. This re-organization of society, rather than one's faith in a religious doctrine, provides the means of salvation. For Undershaft, man does not need redemption from sinfulness but from the material abjection of poverty, hunger, and sickness. The growth of Christian virtues rests fundamentally on man's material security. Undershaft wants nothing to do with a religion that abjures warfare and wealth. These evils are the necessary means by which man can be saved.
Though initially resistant, the democratic Cusins will soon convert to Undershaft's gospel and become his successor. Barbara will return to the Salvation Army with this gospel as well, recognizing that the necessary dialectic between good and evil means that the work of salvation requires the pact with the Devil. They both come to realize Undershaft's power upon the demonstration of his authority over the Salvation Army, a purchase that reveals how all work of salvation is indeed contingent on those who hold the money and gunpowder.
Unlike Undershaft, Cusins takes up the armory in the name of a love for the people. Through the armory, he will abandon his anachronistic and intellectualizing studies and make power for the contemporary world, a power accessible to the masses and that forces the "intellectual oligarchy" to exert itself for the general good. He too exalts the arm as the force that stands to destroy all forces and determines the course of the world.
Undershaft's philosophy also organizes itself around a notion the great man's "will." This will comes into being through the agonistic struggle between men. As Undershaft proclaims, a sacred commandment, "Thou shalt starve ere I starve," sets him on the path to greatness. Through a murderous struggle with others, Undershaft realizes his will and desire. Thus his "bravest enemy" is his best friend, a rival who keeps him "up to the mark." Over and against Christian ideals of human brotherhood, the recognition Undershaft demands from his neighbor is not love but obedience and respect, a bending to his will. Again, the struggle he stages with others is decidedly violent. Those who do not submit to his desire must die.
Such killing in the name of the will does restrict itself to those who stand in the way of the great man's desire, but extant social structures, institutions, and ways of thinking as well. Killing is the means by which the moralist's "ought" becomes a "shall"; it is the "only way of saying Must." Only the murderous command can inaugurate the new that follows necessarily according to the will of the great man. Until he achieves his will, he is menace to civilization; upon its realization, he becomes its benefactor. Thus, the great man makes history.