I think you might be misinterpreting 'act' in this sense - you're looking for 'behave' not 'pretend'.
I behave as though God doesn't exist in the sense that I don't work on the understanding there's another life after this one, that there's an absolute moral authority somewhere to be beholden to, or that I owe obeisance or gratitude to something or someone for my existence.
I operate for my own benefit, and the benefit of those that I personally deem to merit the effort.
O.