It has the same root meaning as infidelity - which is being unfaithful to thy spouse - and of fidelity = being faithful.
An infidel is a person who is unfaithful to their own religion.
As such a Muslim infidel is one who is unfaithful to Islam.
A Christian infidel is unfaithful to Christianity.
It has nothing to do with belief, because an unbeliever is just that = an unbeliever - they are not infidel(s) because if there is no belief or root in the first place then there is no infidelity.
America is some times called an infidel Country and that is because we were known as a Christian Country and now we are unfaithful to our roots, and even aside from Christianity we have an infidelity to God going on.
There is of course a human claim to the fidelity to God (a God by any name or application) and so an opposition to God can make one into an infidel too.
Being anti-God is not the same as an honest claim to being an Atheist, because anti-God would be an infidel to the rest of humanity.
Just saying - FYI.
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