Some opposites of belief:
Disbelief, doubt, nonbelief, unbelief, dissent, uncertainty, distrust, mistrust, skepticism.
The opposite of something that is believed would be something known: fact, knowledge, truth.
Disbeliefs.
non-belifes
atheism
Disbelief - lack of belief or acceptance of something as true.
The belief that the soul and body can't be separated. The opposite, which is the belief that the soul and body are separate, is dualism.
Disbelief. Disbelief: Adj. Lit., does not believe.
disbelief, doubt, unexpected, uncertain, ambiguous
Hypocrisy means saying one thing or holding a belief, and doing the opposite. The opposite of hypocrisy is honesty, straightforwardness, and sincerity.
belief, certainty, confidence, dependence, faith, reliance, trust
You could say atheism but this is not a complete answer. This is a difficult question to answer because not all religions have the same basis. For example is a religion is based on faith or belief the opposite would be a lack of faith or belief, if a religion is based on reasoning or logic then you argue the opposite is ignorance or lack of logic.
The opposite of ethnocentrism is cultural relativism, which is the belief that all cultures have value and should be understood and judged based on their own standards and beliefs rather than comparing them to one's own culture.
Polytheism is the opposite of Monotheism, and is the belief in multiple gods or deities.
The opposite would be believably or credibly. However, the adverb is often used to mean unexpectedly, where the opposite would be obviously or as expected.