Written in the direct aftermath of Undset's conversion to the Catholic Church, the tetralogy presents in a clearly favorable light the Medieval Church with its institutions and rituals; the saintly Bishop Thorfinn of Hamar as well as nearly all priests and monks appearing in the four books are positive characters. The series' central theme is also preeminently Catholic: the tragedy of Olav, a deeply pious and upright man, who feels himself damned and cut off from God because of his unconfessed sin - having secretly killed his wife's lover.
The tetralogy, written after Undset converted to Catholicism, portrays the Medieval Church favorably, emphasizing its institutions and rituals; Bishop Thorfinn of Hamar, along with most priests and monks, are presented as positive figures. The series' central theme is Catholic: Olav's tragic story, a devout man who feels condemned by God due to his unconfessed sin - killing his wife's lover.