It is comparable to the theme (for a work of fiction).
Explanation:
These are all qualities that are present in the controlling idea of a text. This main idea usually includes the topic of a text, and this means that it is usually included in the introduction of it. Moreover, this controlling idea usually represents the underlying message of the text, and it is comparable to the theme in a work of fiction. Finally, the controlling idea is particularly important because this includes the author's viewpoint.