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As Amadeus's one and other answers have pointed, atheists do use religious vocabulary in real life quite often. However, please keep in mind that anything you write can be used to convey some idea ...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/36848 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
As Amadeus's one and other answers have pointed, atheists do use religious vocabulary in real life quite often. However, please keep in mind that anything you write can be used to convey some idea to the reader. If being an atheist is an important feature of the narrator, you can underline that feature by making him not to use religious language at all, specially if you want to contrast him with another character who is religious and often uses religious language.