So, you're the grandchild of Noah. Let's picture the scene:
Life is hard, and you're hungry. Getting enough food from the livestock is a constant balancing act with leaving enough alive to have any herds at all - and, remember, these aren't healthy animals they're infested with every kind of parasite, saved from the flood by the wisdom of God. It's only been a week since the last Triceratops sunk into the mile thick mudflats that still coat half the world, and your idiot father ate the last
Diplodocus before you were born.
So your father swears that that following God's rules saved you all from the destruction and - look - there's a rainbow in the sky to prove it. But let's face it, you're talking about a God capricious and violent enough to decimate the entire world, slaughtering all but a handful of all people, and leaving you eking out this precarious existence with your wife and cousin. Plus your older brothers got all the pretty cousins. You got the one with elephantitis - which God, in his wisdom, chose to save from the flood.
So, yeah, daddy says God is good, and you should serve and worship him, but then daddy's the kind of guy who upon finding his father - your grandfather - drunk thought it would be a good idea to get jiggy with him! Are you really going to trust this guy?
Add it up: precarious, hard existence in a decimated world and a psycho God promoted by your highly dubious family. Seems to me that's a fair breeding ground for a new religion with a little less of the ol' crazy in it. After all, what have you got to lose? If you're wrong he's promised not to do it again, and at least you got to try and do something about those damned parasites.