Kant's quasi-transcendental argument for a necessary and universal evil propensity in human nature

In Part One of Religion within the Bounds of Bare Reason, Kant repeatedly refers to a "proof" that human nature has a necessary and universal "evil propensity," but he provides only obscure hints at its location. Interpreters have failed to identify such an argument in Part One. After examining relevant passages, summarizing recent attempts to reconstruct the argument, and explaining why these do not meet Kant's stated needs, I argue that the elusive proof must have a transcendental form (called quasi-transcendental because Kant never uses "transcendental" in Religion). With deceptive simplicity, the section titles of Part One, viewed as components in an architechtonic system of religion, constitute steps in just such a proof.