Religion is an opinion represented as something else. That something is always supernatural.
A cult is a differing opinion. Usually it forms in reaction to mainstream opinion.
Since both opinions don't really come from anything but imagination, it is a simple matter of interpretative determination.
Think of how this process works out in everyday life.
A family, for instance.
A family has members who must abide by the parent's rule of authority.
At some point there is disagreement and unwillingness to comply.
The dissenting member leaves and starts their own family using a different set of rules.
Cultures work the same way. Political opinions work the same way.
Each group characterizes the splinter dissident as "wrong" and characterizes them in a bad light.
Cults are usually Authority driven to a greater extent than mainstream.
Truly radical cults insulate members against outside opinion and use methods of enforcement which utilize fear and a deep sense of US vs THEM.
Cults are more deeply powered by fantasy than mainstream groups. The stronger the fantasy; the greater the need to act out the fantasy by exaggerated claims.
End Times groups bring fantasy to the edge of reality and dare members to choose as though life or death.
The racheting up of tension between reality and final fantasy ebbs and flows in cults. Old members tire and fall away or find themselves purged and new recruits take their place.
Cults are viral.
The boundry line in cults is extraordinarily defined with rigidity and illogic to the point of mania.