Which of the following best describes someone who believes in one God?

The belief in one God is best described as monotheism. Monotheism is a theological standpoint positing that there is only one divine being who is often viewed as the creator and sustainer of the universe. This concept is central to many major world religions, including Christianity, Islam, and Judaism. In these traditions, the understanding of God typically encompasses unique attributes such as omnipotence, omniscience, and moral perfection.

Henotheism, while related, involves the worship of a single deity without denying the existence of other gods; it suggests a focus on one god among many rather than exclusive devotion to one. Polytheism, in contrast, involves the belief in and worship of multiple gods, each with distinct roles and attributes. Transtheism takes a more nuanced approach, suggesting that the divine transcends traditional theistic definitions but does not necessarily deny the existence of a god or gods.

Monotheism's clarity in asserting the existence of a singular divine entity is what distinctly defines it within the context of religious belief systems.