Why did god allow people to be born after the fall ?

[John V]

Why is there a problem with asking for evidence?

Because the Bible's treatment of the topic is the issue at hand. Whether the bible is demonstrably true or not is irrelevant for purposes of such a discussion. It's fine as a topic of its own, but it has nothing to do with the topic at hand.

Suppose you're in a literature class and discussing The Scarlet Letter. It doesn't matter whether Hester Prynne existed or not. If you were assigned a paper on the themes in the book and wrote There's no evidence Hester Prynne existed, so there's no point in addressing the themes, you'd fail.

Or, as noted, when an atheist makes a post alleging a problem with the Bible, other atheists don't jump down his throat demanding evidence for the Bible. That typically happens when the atheist starts losing to Christians.

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Sin doesn't exist outside of religion,

Considering that we're in the Christianity (a religion) forum, which is a subforum of the Religion forum, it's an appropriate point of discussion, don'tcha think?

Your point would be appropriate for the Philosophy section.