Demonization is the reinterpretation of polytheistic deities as evil, lying demons by other religions, generally monotheistic and henotheistic ones. The term has since been expanded to refer to any characterization of individuals, groups, or political bodies as evil.
Religions, even those that are radically monotheistic, do not necessarily deny the existence of other gods or spiritual beings. On the contrary, they claim other gods are not worthy of worship and in actuality are demons who mislead followers from proper belief or practice. Christian missionaries often employed demonization tactics when converting pagans, although Judaism, Islam, and other religions have similar histories. Demonization is not limited to focusing on other religions but can also be directed inward to condemn various schools of thought or movements.
From a secular viewpoint, demonization can be used to denigrate an opposed individual or group, making adherents to your own religion or viewpoint less inclined to do business with them (and possibly convert) and more inclined to fight against them. If foreigners are evil and corrupted by demonic influence, then any means of self-defense is easily portrayed as legitimate. The portrayal of almost all pagans in the Middle East as Baal-worshippers in the Hebrew Bible is an example of this. If pagans are corrupted by the demon-"god" Baal, then clearly they must be fought or at least oppressed. Especially in the earlier books of the Hebrew Bible, foreign deities are portrayed as existing and corrupting entities rather than being mere powerless idols. Some would argue this later transferred to Christianity after Constantine I's ascension in its suppression of Roman paganism. Much later, the language of demonization would be invoked during the Spanish Inquisition, leading to the expulsion of Jews and Moriscos from Spain.