Campbell, Lily B. "Doctor Faustus: A Case of Conscience." Modern Language Association. 67.2 (1952): 219-239. JSTOR. Web.
Campbell suggests Doctor Faustus to be a sort of morality play warning against the sin of despair. She suggests that the audience doesn't Faust's sin as being obvious (in contrast to Goethe's interpretation of the legend, wherein Faust seduces a young maiden, Gretchen). Campbell suggests that Faust's sin is two-fold, with the later being of greater importance but needing the first. His first sin is selling his soul to the devil, this sin leads him to his second, but more dangerous sin, that of thinking he is beyond salvation: Despair. In this way, Faust exemplifies a kind of Rennaisance Humanism that centers around man instead of God, making man self-sufficient.
The author posits that we more background to see just how clear this theme of despair is. This understanding requires an understanding of justification by faith (that is, forgiveness for all sins by faith in God alone) as opposed to the Catholic doctrine which holds justification through works and avoiding grave sin. She suggests Marlowe's expression of the Faust legend as clearly analogous to a real-life case of conscience that theologians would have been discussing at the time, and Marlowe surely would have been taught about in his religious education: that of Francis Spira. Spira denounced his Protestant faith for fear of Catholic oppression. This denial brought about a struggle of conscience which is similar to Marlowe's Doctor Faustus', most interestingly in Spira exclaiming he could tolerate hell if he had some hope for its end, an exclamation echoed by Faustus. Spira became a sort of archetype of the man in despair at the time, and Marlowe would have known this. Campbell suggests that the similarities between Faustus and Spira are deliberate, and that this certainly suggests that Marlowe was writing a play expressing the consequences of despair.