In the New Testament God does not speak of the faults of the saints in the Old Testament, such as Abraham and Sarah. It’s as if God is telling us “Don’t you remember their sin! Don’t recall the unbelief!” This is massively encouraging. As Christ said of the disciples, when He prayed to the Father in John 17:6, “they have kept your word” — despite their unbelief and failures in the past.
By faith Sarah herself received power to conceive, even when she was past the age, since she considered him faithful who had promised. – Hebrews 11:11