The Fast Fall from Faith to Fretting

Check this out.
 When Abraham was just plain old Abram, God promised him a child. The dude's old at this point. Way more likely to be kicking it with great grandpas than first-time fathers.

But God promises him a child and Abram believes.

Abram believed the Lord, and the Lord counted him as righteous because of his faith.-Genesis 15:6
 Although this promise was by all accounts impossible, Abram believes and God declares him righteous because of his faith. 
 Pretty awesome, right?
 Yeah. Until...