I wonder why the British novelists who are passionate about religion seem to be mostly Catholics (yes, thinking of Waugh here). Is it perhaps too difficult to be passionate about the C. of E.?
Anyway, Greene always raises the questions which can be perfectly well posed and thought about by atheists; and while it's fairly obvious that his personal answer to them is religious, he never shoves it down the reader's throat.
Other than that, I'm not ready to analyze the literary qualities, because I'm too much in tune with Greene to be objective and rational. I often find myself understanding what he says before he's half finished. That's a treasure to find, but it cannot be passed on.