The only difference between a cult and a religion is the number of people that believe what is been taught is true. So all religions start small and then get bigger at some point. So the logical rationale is that all religions started out as cults. By the way I am not proclaiming this as my own idea, this idea has been around for decades at least.
All religions start with a small following, sans toughs that are mandated by kings. It is only a cult if it is called a cult anyways, it is a rather subjective term. For that reason it is possible to call Christianity a cult, even though it is now a mainstream religion.