At a glance, only size seems to be the immediate difference between an established mainstream religion and a corresponding cult. This might be the only difference actually, because the other differences between the two only exist when the State exists.
Religion needs the patronage of the State, while the cult doesn’t. The former has some relationship with the State, while the other is relatively free from it. Religion, by its mere existence, influences and controls the State; but ‘modern secular democracies’ pretend that religion is not of importance, and its time of importance is long gone. Yet they continually bend over and soften to one religious view or the other whenever and wherever it is strategically necessary. Religion, even though presented as a weakened force in front of the new faith of consumption and science, remains at the core of most political and social issues.
But, a cult is largely absent from the gaze of the State, atleast until it becomes large enough to demand the same power and acknowledgment which is given to a state-recognized religion. Usually that is the moment a cult makes it to the news. This is also why the State is wary of cults and preaches so passionately against them as something dangerous. Cults interrupt that existing balance of power between the State and the Old Church.
Though religion has always been effective in consolidating and enforcing political power, like all things organic and human, it cannot remain stagnant and it ends up budding new interpretations, reformations and counter-narratives. These are the new cults. All they then have to do is to weather the isolation and the persecution till they grow in numbers large enough to dethrone the old stale host they gained most of their philosophical or spiritual nourishment from. As the State recognizes this new religion, the cycle begins again. Religions are nothing but State-sanctioned cults.