Did Sarah believe she would conceive?

Both texts are true, at different moments. Genesis 18 records Sarah’s first reaction. She was ninety and long past childbearing (17:17; 18:11), and she heard the promise from inside the tent, spoken by a guest whom she may not yet have known to be the Lord (18:10). Her laugh was doubt, and the Lord rebuked it: “Is any thing too hard for the Lord?” (18:14). Her denial came from fear, and Genesis records it without excuse.

Hebrews speaks of what followed. The promise was repeated with a set time (18:14), and Sarah bore Isaac “at the set time of which God had spoken” (21:2). Hebrews says that she conceived because “she judged him faithful who had promised”. The writer is not denying her laugh; he is saying that she came to trust the promise she had first doubted. Her words after the birth show the change: “God hath made me to laugh, so that all that hear will laugh with me” (21:6). Abraham too had laughed at the promise (17:17), and Hebrews 11 is full of people whose faith had failings, such as Gideon, Samson and Jephthah (11:32). It praises their faith, not their weakness.

Hebrews speaks of Abraham’s faith

Some read Hebrews 11:11 as praising Abraham’s faith, with Sarah’s barrenness as the obstacle it overcame, since the verses around it are about Abraham and verse 12 speaks of one man. Several modern translations render it that way, and some manuscripts add that Sarah was barren. John Gill notes this view but prefers to read the verse of Sarah, who ‘though, at first, she distrusted, yet she afterwards feared, and believed’.

Genesis does not record the moment when Sarah came to believe. Whether Hebrews 11:11 is about her faith or Abraham’s turns on a point of Greek grammar on which translators are divided; either way, the verse does not deny that Sarah first laughed.