Most personally convincing reasons you don't believe.

[Wyrd of Gawd]

As a kid, I loved to read Norse and Irish mythologies.  As much as I enjoyed the stories, I knew - on a gut level - that they weren't true.  There was no Thor, no Loki, no Cúchulainn and no Ferdiad.  But when it came to Bible stories, I was expected to believe in David, in Sampson, in Jacob, and scads of tales of mighty men, magic spells, terrifying adventures, and so forth.

So, I suppose what convinces me most, on a personal level, is that I honestly can't tell the difference between a mythic tale and a 'sacred' text.  I know that Joshua didn't destroy Jericho with trumpets in exactly the same fashion that I know Fergus of Ulster didn't cut three hills in half with his sword.

Boru

The Bible itself says that the stories are primarily entertainment and not real events.  After all, how insane do you have to be to believe in dialogue where there's just one character?