The abstract noun "faith" can be expressed with the verb "believe." Possible meanings are 1) it was by Abraham's faith. AT: "It was because Abraham believed God" or 2) it was by Sarah's faith. AT: "It was because Sarah believed God" (See: [[rc://en/ta/man/translate/figs-abstractnouns]])
This means that just as there are so many grains of sand on the seashore that no one can count them all, Abraham had so many descendants that no one can count them all.