When a parachutist stands in the door of an airplane ready to leap into the air, he does not have faith that gravity will pull him to earth. There is no flicker of doubt that he might instead fly. So, there is no ground for faith on that point.
But it is only his faith in his parachute that gets him to step into the air and leave the plane behind. There is an unavoidable doubt about it all. Parachutes sometimes don’t open. Things go wrong. There is an unresolvable germ of doubt that he cannot eliminate. So taking that step is an act of faith.