Your grandfather is an exception to the rule.
But what arguments specifically are you looking to debunk quickly?
The people down south are not smart. I am not expecting deep, convulted logical loops ab out the creation of the universe.
Stick with the Bible. Things like, how the Bible says that God says that there is proof in nature alone to acknowledge his existence, yet God also says you must forsake logic in favor of blind faith, yet God also expects that the uninformed people of non-Christian areas discover Christianity through nature, which would be a logical exception -- etc.
Also how God loves you but will send you to hell. How God is an omnipotent being and can see the future, but we also have free will. Strong "wtf?" things like that.