The sentiment behind the quote I’ve made the title of this post is an oft-repeated sentiment (although, this exact wording I’ve just taken from an interview between Sam Harris and Bill Maher). And the sentiment can be reversed; every Muslim is an atheist with respect to Christianity. The sentiment is often followed with words akin to atheists just go one God further. But I do not approve of this sentiment or quote.
Whether you are a monotheist, polytheist, a follower of one of the Abrahamic religions or an Eastern religion you share something in common with all the other theists: you believe in some core sense of a God. There are some details and addendum that then make up your specific religion; and not all religions are equal. But you share a belief in a God. This is important, because in theory you could convert from Christianity to Islam, or from Judaism (the religious bit of it, not the bit that is defined by inheritance) to Jainism, or from Hinduism to Cao Daiism. And to do this you would not have to shift in the idea that a god exists. You could keep up a belief in an afterlife. It would be easy to ascribe to an objective morality by the same method. Your purpose and the origin of the universe, more or less, would be unaltered by jumping around different religions.
It is significant to then exit all religions. You have to admit that you don’t know where the universe came from (yet); you have to admit that morality, objective or otherwise, is a construct that emerges only after consciousness is recognised; the afterlife is no longer an option. Knowing that pretty much all apologetics could be used to defend pretty much any religion (although, I believe they defend religion badly) it becomes apparent that all religions are more similar to each other than absence of religion is to any of them.
When you say that a Christian is an atheist with respect to Allah you ignore that fact that part of the reason they reject Allah is precisely because they have accepted Jesus. There is still a religious element to it. The religions are more similar than either is to complete absence.
There is one sentiment I do wish to salvage from this quote though, and it is this: once a person has accepted a particular religion they view other religions with the same cynicism and scepticism as an atheist would view it. But they defend their own religion from the same scepticism they lend to the religion of others.