Why is the novel divided into three parts: mosque, caves and temple? What is the novel describing?

Forster uses the titles of the sections to foreshadow what is to come in each part. Each section focuses on a specific group of people, a specific season, and a certain kind of behavior in each of the three sections.

In "Mosque" the focus is on the Muslim characters. Recall that we meet Aziz and his friends in Chapter II; Aziz meets Mrs. Moore in a mosque. The season is spring, a time of cool weather in India; the behavior will be characterized by relative restraint and sanity.

In "Caves" the British are the major characters. The hot weather of summer is associated with irrationality, nightmares, hallucination, and visions of cosmic disorder.

In "Temple," the final section, the season is autumn, which is rainy in India. The setting focuses on a Hindu festival (the birth of Krishna); rains accompany revival and refreshment, a renewal of the earth and of life itself.