What religious belief emphasizes the existence of only one god?

Monotheism is the belief system that specifically emphasizes the existence of only one god. This concept is foundational to several of the world's major religions, such as Christianity, Islam, and Judaism, each of which professes a singular divine authority. Monotheistic faiths focus on the idea that this single god is sovereign, omnipotent, and possesses ultimate authority over creation and moral law.

In contrast, polytheism involves the worship of multiple gods, each often overseeing different aspects of life and the universe. Agnosticism represents a philosophical stance on the knowledge of the divine, suggesting that the existence or nonexistence of gods is unknown or inherently unknowable. Atheism, on the other hand, is the disbelief in the existence of any gods.

Thus, monotheism stands distinct in its exclusive focus on a singular divine entity, making it the correct answer in the context of the question.