Alice C. Linsley
A Man Called Thursday is a typical Chesterton piece of paradox. Sunday is a Christ figure who puts us through our paces. Chesterton has Thursday quote Job at the beginning and end of the book.
We don't trust him (Sunday) and like the Secretary, we have trouble forgiving him for the peace (of the Sunday/Son's day). We fear him. We think we can run away from him. Yet he is the one behind the big plan.
We have a clue about Sunday's identity when Thursday says that Sunday's face frightened him because it was so good. There is a sense of knowing the Sun's existence without being able to look directly at the Sun. By looking at Thursday we are able to "see" Sunday hidden in the periphery, a mysterious overshadowing light.
G.K. Chesterton's brother Cecil Chesterton believed that Sunday was patterned on F.W. Walker Oddie, the High Master of St. Paul's Day School, where Chesterton attended. He was both loved and feared for his "mere bigness and irresistible natural power." (John Milton also attended that school.)
Related reading: Chesterton on the Value of Detective Stories