Cults that are widely successful to become Religions. Religion looks unfavorably towards Cults, recognizing that they are the competition.

A cult usually has a leader and is limited in size, while a religion is a larger movement which has iconography, art styles, some kind of scripture.

From a sociological viewpoint a 'church', to use use an over-arching term for conformed religious practice, can splinter into sects that are a protest against the authority of the mainstream doctrine.

The main difference between sects and cults is that sects are usually seeking a return to the 'purer' or 'more authentic' roots of their religion. A cult offers something new or a discovery of something which has been perceived to have been 'lost'

Interestingly, in the field of Religious Studies, cults are beginning to be called New Religious Movements or NRMs due to the term 'cult' being deemed pejorative.

Here's a list of traits shared by most cults, as compiled by some Ph.Ds that study cults and their behaviors. I'm sure you can adapt them to all kinds of political groups and other forms of ideology. If your question was "when is a religion not a cult", my answer would simply be: never.

The group displays excessively zealous and unquestioning commitment to its leader and (whether he is alive or dead) regards his belief system, ideology, and practices as the Truth, as law.

‪ Questioning, doubt, and dissent are discouraged or even punished.

‪ Mind-altering practices (such as meditation, chanting, speaking in tongues, denunciation sessions, and debilitating work routines) are used in excess and serve to suppress doubts about the group and its leader(s).

‪ The leadership dictates, sometimes in great detail, how members should think, act, and feel (for example, members must get permission to date, change jobs, marry) leaders prescribe what types of clothes to wear, where to live, whether or not to have children, how to discipline children, and so forth).

‪ The group is elitist, claiming a special, exalted status for itself, its leader(s) and members (for example, the leader is considered the Messiah, a special being, an avatar the group and/or the leader is on a special mission to save humanity).

‪ The group has a polarized us-versus-them mentality, which may cause conflict with the wider society.

‪ The leader is not accountable to any authorities (unlike, for example, teachers, military commanders or ministers, priests, monks, and rabbis of mainstream religious denominations).

‪ The group teaches or implies that its supposedly exalted ends justify whatever means it deems necessary. This may result in members' participating in behaviors or activities they would have considered reprehensible or unethical before joining the group (for example, lying to family or friends, or collecting money for bogus charities).

‪ The leadership induces feelings of shame and/or guilt in order to influence and/or control members. Often, this is done through peer pressure and subtle forms of persuasion.

‪ Subservience to the leader or group requires members to cut ties with family and friends, and radically alter the personal goals and activities they had before joining the group.

‪ The group is preoccupied with bringing in new members.

‪ The group is preoccupied with making money.

‪ Members are expected to devote inordinate amounts of time to the group and group-related activities.

‪ Members are encouraged or required to live and/or socialize only with other group members.

‪ The most loyal members (the true believers) feel there can be no life outside the context of the group. They believe there is no other way to be, and often fear reprisals to themselves or others if they leave (or even consider leaving) the group.

[csj.org]

Some interesting replies, most seem to think it's to do with it being an individual at the centre, surely that's how all religions started?
If it's a numbers game then surely that's down to longevity of the religion / cult?

When we watch The Rocky Horror Picture Show or Monty Python And The Holy Grail.

A cult is usually started by an individual who has been in a religion and didn’t like having to obey the rules so he starts his own. It is basically the same as the one he has left, but he places himself at the head of it and names it after himself. if he is a real megalomaniac he will also replace god with himself. He will also invent some new weird rituals, the weirder the better, because thats what attracts the most gullible.

Or when a religion not a cult? My guess is that difference may lie in the degree of overt control the organization exerts on its individual members.

When is a religion NOT a cult?

Outside of "religious cult", a cult is not a religion, it seems. The tems don't conflate at all.

Yeah when it doesn't have enough followers to pull any serious influence.

There's an old saying that the difference between a religion and a cult is:

At the center of every cult there is at least one person with direct experience that the whole thing is bull -- whether they admit it or not.

In a religion that person is dead.

I heard a discussion on NPR with a priest, a rabbi, and a minister about that very subject. In the end the only thing they could come up with was size.