Read Genesis 17
Focus on verse 17-19
The name Isaac means “he laughs”. Abraham’s son by Sarah would be called Isaac because when God told Abraham that Sarah would have a son he “fell on his face and laughed”. In the next chapter we will find that when Sarah overhears the angel of the Lord telling Abraham this she also laughs (and then denies laughing)
Then Abraham fell on his face and laughed and said to himself, “Shall a child be born to a man who is a hundred years old? Shall Sarah, who is ninety years old, bear a child?” (v. 17)
When Abraham suggests that Ishmael could be the child of promise, God responds quite directly saying “No, but Sarah your wife shall bear you a son,” (v. 19) adding that He will also bless Ishmael, but the covenant will go forward through Isaac.
God has not changed in the way he communicates and blesses His people. It would seem the nature of humanity has not changed much either. I remember the first time someone suggested (seriously) that I should consider ministry as an occupation… my response was similar to Abram’s… I laughed! This was followed not long thereafter by my spending the better part of 20 years in full time church ministry! This is not that surprising to me when I think about it now… the result was that I was rarely doubtful of my calling since it was so obvious to me that it was God who put me there.