”By faith Sarah herself also received strength to conceive seed, and she bore a child when she was past the age, because she judged Him faithful who had promised.” (Hebrews 11:11; Gen. 18:10-15)
This passage makes it sound like Sarah never had any doubts, just as the previous verses seem to imply that Abraham never had any doubts (Hebrews 11:8-12, 17-19). Yet when you go back and read the Genesis accounts, Abraham, out of fear, said that Sarah was his sister (Gen. 12:10-12), and Sarah laughed in unbelief when she first heard that in her old age she would have a child. Under the new covenant God remembers our sins and failures no more!
”This is the covenant that I will make with them after those days, says the LORD: … “Their sins and their lawless deeds I will remember no more.” (Hebrews 10:16-17; see also 8:8-13; Jer. 31:31-34).