Aneeda, the problem with your question is that it's awfully general. There are many religions, past and present, on earth. Saying that anything is true about "all" religions just invites the question, "Yes, but what about...." Judaism is not a cult, although there may be small fringe groups that fanatically follow one rabbi, and therefore, maybe they are. How about nature worshippers? And believers in all sorts of deities who are not human? Some people get religious about the zodiac, for example.
Whether the definition of "cult" has to say something about it being a small group is a good question. How small is small? There are all sorts of groups on earth that could be called cults because they fanatically follow a particular leader. Yet, they don't have anything that most of us would call religious practices. So IMO, all religions are not cults, and all cults are not religions. Though obviously, some are both.