The outsider test is a criterion for rational belief developed by former Christian apologist John W. Loftus. Loftus observes that religious affiliation is largely determined by that of one's parents and native country, and to counteract this tendency one should "Test your beliefs as if you were an outsider to the faith you are evaluating."
The outsider test codifies a form of argument that has existed in critiques of religion for some time.
In Bertrand Russell's speech "Am I An Atheist Or An Agnostic?", Russell said that he could not prove there was no God, but could not disprove the existence of the Homeric gods either. More recently, Richard Dawkins argued "We are all atheists about most of the gods that humanity has ever believed in. Some of us just go one god further." A similar slogan, coined by Stephen Roberts and used in many internet taglines, says, "I contend we are both atheists, I just believe in one fewer god than you do. When you understand why you dismiss all the other possible gods, you will understand why I dismiss yours."
The basic idea of the outsider test has also been applied to questions more specific than the existence of God. For example Richard Carrier, in an article on Jesus' resurrection, argued "Can you imagine a movement today claiming that a soldier in World War Two rose physically from the dead, but when you asked for proof all they offered you were a mere handful of anonymous religious tracts written in the 1980's? Would it be even remotely reasonable to believe such a thing on so feeble a proof? Well — no."
In his Natural History of Religion, David Hume gives a long discussion of how the beliefs of one religion may appear absurd to another, though he draws no explicit conclusions from this fact. Included in this discussion is an imagined exchange between a Catholic and a pagan:
"How can you worship leeks and onions? we shall suppose a SORBONNIST to say to a priest of SAIS. If we worship them, replies the latter; at least, we do not, at the same time, eat them.* But what strange objects of adoration are cats and monkies? says the learned doctor. They are at least as good as the relics or rotten bones of martyrs, answers his no less learned antagonist. Are you not mad, insists the Catholic, to cut one another's throat about the preference of a cabbage or a cucumber? Yes, says the pagan; I allow it, if you will confess, that those are still madder, who fight about the preference among volumes of sophistry, ten thousand of which are not equal in value to one cabbage or cucumber."
*This is a reference to the doctrine that in communion, the bread literally becomes the body of Jesus, whom Christians worship, putting them in the position of eating their god.
Loftus, John W. Why I Rejected Christianity: A Former Apologist Explains Tradford 2006.