The burden of proof rests on one who asserts something is true. If something is not true, it is then by default false. A person claiming something is false has no burden of proof to uphold, the person stating something is true must show that their assertion is incorrect.
Anyhow, the reason I think Christianity, or for that matter, any particular specific religious philosophy or belief is illogical, is that there are many out there, and many different places where the word of what ever deity/deities really do exist could have been corrupted or falsified. Certainly, there are many logical arguments against atheism (Note: Anslems Ontological argument is not one of them
), but nothing to suggest that any of specific belief is correct.
For instance, Christianity, since that's what's being argued. Off the top of my head, there are...four possible places where the word of god could have been falsified.
1.) Interpretation. Simply put, there IS room for interpretation in the bible. Did god really reverse the rules for eating "unclean" foods, or was that one guy just special? Many places in the Bible are up to interpretation, so these things are up for interpretation. (Which is how you can have many people following the same book as though it was the word of God, and still have wars over it).
2.) Translation. Yes, I know, it's a tired example, but it's true. Someone could have written something incorrectly when they were translating it from Hebrew to whatever. Not only that, but someone could have written something slightly incorrectly when they were copying it from one book to another. It's a very old religion, so that leaves alot of room for error. With that much room for error, it's certainly logical to still follow the faith, but not to say you're 100% certain.
3.) Recording. Who heard the word of God? Who wrote it down? People. Fallable people, imperfect people. Who knows whether God really said pigs were unclean, or that homosexuals were sinners? Not only could someone have misheard, or copied it incorrectly, but a homophobe, or someone with a pork allergy could have added that little bit in there. See what I said for translation, there's no way you can logically say you're 100% certain that Christianity is correct.
4.) God could have lied. Maybe a supreme being does exist. How do we know the one that came to earth was telling the truth? It could have been an evil being, tricking us. There's no way to tell that any being who claims to be divine is telling the truth. It's not logical to assume that you think one religion is true when there's no logical way to tell that a) the deity really said what you believe b) that the deity wasn't lying, or c) that a deity even exists!