Is religious an antonym for spiritual?

Answer 1

No. Someone might say that in a snarky or sarcastic way if they don't like organized religion, but it is not an antonym in the language, and religious people are very often, but not always, spiritual.

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Not necessarily.

Those who call themselves spiritual are often not affiliated with a religion. There are often things about structured religions that make these people uncomfortable or that they do not agree with. Instead of following a set religion, they choose to believe in a higher power but not worship in an institution that gives them a set list of what to believe. It's not the same as being atheist or agnostic.

Whereas, those who are religious are generally affiliated with an existing religion and follow its beliefs.