|Practical arguments (Photo credit: Wikipedia)|
Briefly, an ex post facto argument is a logically flawed claim in which claimant begins by assuming that an unsupported claim is true and then reasons backward from the claim in order to support it. Students of logic will immediately recognize the problems with such an approach. Instead of offering premises which lead to a conclusion, the ex post facto claim begins with the conclusion, assumes it to be true without having demonstrated it as such, and then attempts to manufacture support for the conclusion. The merit, falsifiability or, or veracity of the conclusion is never questioned.
When Christians attempt to use reason to support their claims, they do so in an ex post facto manner. They start by accepting the conclusion that their god exists and then try develop what appear to be reasoned arguments to support this conclusion. This violates the most rudimentary principles of logic, ensuring that the result will be irrational. They are claiming to support the claim that their god exists, but this claim is never actually up for debate; it is the core presupposition from which the argument begins.
Another striking and extremely common example of ex post facto reasoning can be observed whenever a Christian commits a heinous criminal act (e.g., pastors molesting children). The unsupported conclusion is something like, "Christians don't do bad things." From this point, you hear them work backward to deny that the perpetrator was a "real Christian." What is never questioned is the claim that Christians don't do bad things.
As Nathan points out, ex post facto arguments should not be accepted. They are irrational on their face and should be identified as such. Beginning with the assumption that some god exists without first proving it is a logical fallacy.