Hebrews 11:11 is exhibit 999 that in the Bible true faith is not perfect faith.

By faith also Sarah, though barren, received power to give birth, even in old age, because she considered the one who made the promise to be faithful. (Hebrews 11:11)

Sarah’s faith was far from perfect. Sarah was, in fact, rebuked for laughing at God’s promise to her and Abraham. She even tried to deny it when confronted with her disbelief (Genesis 18:13–15).

Hebrews makes clear that, in the end, Sarah’s faith was real and sufficient to be commended: In the end, Sarah considered the one who made the promise to be faithful (Heb 11:11).

Real faith is not perfect faith. All that’s needed is a mustard seed’s worth.