Hearing a promise and accepting it on the spot is one thing; waiting around for almost 20 years for it is another.
Although Abraham had what we now call a righteous response to God’s promise of a son, the waiting proved to be hard. The reminder came 15–20 years after the first promise. By then, Abraham was 99 and Sarah (his wife) was 90, and he gave a very human, and less-than-perfect, response:
17:17 Then Abraham bowed down to the ground, but he laughed to himself in disbelief
God’s not stupid. He knew Abraham laughed, but rather than give up, God corrected him and repeated the promise, making it clear than the 90-year-old Sarah would indeed be the mother. Abraham must have taken it on the chin, as he had all the males in his family circumcised as a symbol of that promise, as God had directed.
Shortly afterwards (possibly the same day), God told Sarah the same thing. She laughed, too, leading to a No, I didn’t – Yes you did exchange between Sarah and God:
18:13-15 Then the Lord said to Abraham, ‘Why did Sarah laugh? Why did she say, “Can an old woman like me have a baby?” Is anything too hard for the Lord? I will return about this time next year, and Sarah will have a son.’ Sarah was afraid, so she denied it, saying, ‘I didn’t laugh.’ But the Lord said, ‘No, you did laugh.’
Still, God didn’t give up on them, and Isaac was born within a year.
Paul later tells us than Abraham never wavered in believing God’s promise. It means a moment of laughing in disbelief or trying to make God’s promises happen ourselves (as Abraham and Sarah did through Ishmael) doesn’t disqualify us from God’s promises.