You can do very little with faith, but you can do nothing without it.
Guest Passes let you share your photos that aren't public.
Anyone can see your public photos anytime, whether they're a Flickr member or not. But! If you want to share photos marked as friends, family or private, use a Guest Pass.
If you're sharing photos from a set, you can create a Guest Pass that includes any of your photos marked as friends, family, or private. If you're sharing your entire photostream, you can create a Guest Pass that includes photos marked as friends or family (but not your private photos).