The concept of redemption can also be a theme. Maybe characters in the 'Speak of the Devil Novel' have the chance to turn away from the influence of the devil and find a better path. Additionally, the power of belief is important. Whether it's believing in good to overcome evil or having faith in something that can resist the devil's wiles. There could be a theme of secrecy too, as the devil often operates in the shadows, and there are likely hidden motives and actions in the story.
The theme of morality is likely present. 'Speak of the Devil Novel' may question what is right and wrong in the face of the devil's influence. It could also touch on the theme of fate. Are the characters doomed because of the devil's interference, or do they have the power to change their destinies? And the exploration of human nature is probably there as well, seeing how people react when faced with the devil's temptations and the darker side of existence.
One main theme could be the struggle between good and evil. It often explores how the devil or devil - like characters interact with the world of the good. Another theme might be temptation, as the devil is often associated with leading people astray.