I do not see why religions should be treated differently than secular ideologies. Would it be wrong to offend Fascism or Communism if Mussolini or Marx claimed divine inspiration?
It depends on the context. If someone is offended by the idea that you don't believe in god, that does not make it wrong to not believe in god. If you're intentionally offending someone as a method of signalling your loyalty to the atheist crowd or to some other religion by burning your bridges with that one, I think it's a bad idea. It's not good for all the subcultures to hate each other.
It is stupid and I don't see much point what could be achieved by purely offending anybody. Freedom of religion is a good thing but it doesn't include a demand to respect any religion just not interfere with its practise.
Most often offence happens when a separate more important need is trying to be met. Some people find it emotionally hard to be critical without being offensive/hostile. Sure it would be a positive move to have critism not be offensive but usually the need to articulate clearly the critism is more important than having it in a politically correct form.
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