Ether 12:7

for it was by faith that Christ shewed himself unto our fathers, after that he had risen from the dead; and he shewed not himself unto them, until after they had faith in him; wherefore it must needs be that some had faith in him, for he shewed himself not unto the world.

It was by faith that Christ showed himself to our fathers after he rose from the dead. He did not show himself to them until after they had faith in him. So some people had to have faith in him first. He did not show himself to the world.

It was through faith that Christ revealed himself to our ancestors after he rose from the dead. He didn’t show himself to them until after they had faith in him. That’s why some people had to believe in him first—he didn’t appear to just anyone.