Donat plays the classic English parson, a role unchanged since Trollope, poor, scrimping, of moderate talents but immense goodness. When forced to face his own mortality, he becomes happier than ever before, since he can act with his beliefs out there for all to see.
The film also addresses the very common idea that a life of religion is one of rules alone, and demolishes it brusquely. The religious life is not one of rules but one of freedom. Freedom from many things, but freedom to do others. It is compellingly summarized in his brief but heartfelt sermon that is eagerly misinterpreted by the masses. But it is the message Jesus offered 2000 years ago. If you believe, and act on that belief, rules no longer are important. That is the ultimate freedom, and why Donat can be so happy while under a death sentence.
Fine film, understated yet potent.