Abraham (then known as Abram) was probably around 80 years old when God promised him a son, and more descendents than he would be able to count. Abraham’s response is one of the great examples of how we can’t earn righteousness by doing anything. We just have to believe God’s words:
15:6 And Abram believed the Lord, and the Lord counted him as righteous because of his faith.
Paul later tells us than “Abraham never wavered in believing God’s promise“. It was a rougher path than than, and it means that a moment of doubt doesn’t disqualify us from God’s promises.