The abstract noun "faith" can be expressed with the verb "believe." Possible meanings are 1) it was by Sarah's faith. Alternate translation: "It was because Sarah believed God ... she" or 2) it was by Abraham's faith. Alternate translation: "It was because Abraham believed God ... he" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])
Possible meanings are 1) Sarah received the ability to conceive a child. Alternate translation "she became able to bear a child" or 2) Abraham received the ability to father a child. Alternate translation: "he became able to have a child"
"because she believed God, who had given the promise, to be faithful." Possible meanings are 1) Sarah considered God to be faithful or 2) Abraham considered God to be faithful.