What are the main themes in the novel, The Keys of the Kingdom?

In The Keys of the Kingdom Cronin is concerned with two main themes, one of which is a constant concern throughout his life and work, while the other was never to appear again with such fervor. The first is Cronin's concern for children who are ill-treated or neglected. As is often the case in Cronin's novels, children also have great potential intellectually and socially.

The second theme is theological. Francis becomes a priest, but from the time he begins his training, he finds himself at odds with his superiors and peers in theological stances. He announced to a teacher, "Surely, sir, doctrine is such an accident of birth that God can't place an exclusive value on it." The statement typifies the faith that Cronin, in this novel, would proffer. It is a faith opposed to organized religion, but only insofar as it is harmful to faith.