LAUGHING AT GOD S1e90 Gen 17:15

Both Abraham and Sarah Laugh at God regarding His promises for children. In my view, it is not their finest moment of faith.  They both walk-back their comments. 

God doesn't let it go. He reminds them of this moment by actually NAMING this promised child ISAAC, which means HE LAUGHS.  So, every time they call Isaac to dinner they are reminded of this time.  Fortunately, God is the one that is faithful.
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