Abraham’s reaction to God's promise of a son by Sarah is best described as faithfulness because he believed in God’s word despite his old age and Sarah's barrenness.

In Genesis 15:6 (RSV), it is written:
"And he believed the Lord; and he reckoned it to him as righteousness."

This verse shows that Abraham trusted God’s promise completely, even though it seemed impossible from a human perspective. His faith was later reaffirmed in Romans 4:20-21 (RSV):
"No distrust made him waver concerning the promise of God, but he grew strong in his faith as he gave glory to God, fully convinced that God was able to do what he had promised."