Moreover, if your definition of "cult" is a group with a charismatic and very odd leader who thinks he or she has direct access to the divine and spreads a theology that seems both heretical and confused to the established religions around it, then Christianity and Islam and Buddhism were certainly cults when they began — and no doubt the Jews were as well.
Here, I suggest, is the real difference between a cult and a religion: about 100 years. Once a cult is able to establish itself for several generations, we call it a "religion." Before that, we dismiss it as a dangerous threat to real religion.
This may seem a mocking, cynical dismissal of the difference, and hence of religion itself. But I don't mean it that way. For there are good reasons to respect a group that can maintain a vision of how to live across two or three generations, ones that do not apply to groups that come and go within a single generation.
To start with the most obvious points, a group that survives over generations cannot afford the sort of self-destructive, oppressive or anti-social behavior that appalls us in cults. It cannot engage in mass suicide, of course, nor is it likely to continue if it prescribes extremely unhealthy practices. And it is likely to fall apart, or draw upon itself harsh attention by the political authorities around it, if it oppresses its members or engages in attacks on outsiders. To become a religion, a group with a shared vision of what God wants, or what makes human life worth living, is therefore likely to develop a morality much like that of the society around it — and indeed declare that morality central to what it has to teach.
A group that survives over generations will also have to develop institutions for teaching its message to its young. But no system that has horrific or very bizarre implications is likely to retain the loyalty of its young (they, after all, do not join the group out of some unusual personal experience: born into it, they need to be persuaded of the group's message in a very different way from their parents). Nor is it likely to inspire a cadre of teachers or enable its educational institutions to solve their administrative and interpersonal issues harmoniously.
Finally, a group that survives over generations will normally need to reconcile its religious message enough with what the rest of the society around it believes and does that its members can find jobs in that society, maintain neighborly and economic relationships with others in that society, and function as citizens. All this requires that it temper or reinterpret the stranger claims and practices of its founding generation.