Truth is what reflects reality, not the words for those things. As always, one must look past words to reason and ideas when discovering or savoring a truth. This is as much the case with the nature of cults and religions. What distinguishes one from the other? Some religious people might consider every religion other than theirs, as well as the adherents, cultish simply because it is not their own, though this is an arbitrary and self-based approach to realities that do not depend on perception or preference. Others might consider a cult a group that is bent on some destructive end or think of a non-mainstream religion as a cult, whereas they would think of something like Christianity or Islam as a religion. Is it mere popularity that differentiates the two?
Many examples of what are called cults relate to the following of a charismatic, secretive, or powerful individual, such as Jim Jones of Peoples Temple and Bonnie Nettles of Heaven's Gate. In some cases, they might be devoted to syncretism of a dominant religion like Christianity, or at least the cultural distortions of it, and some extraneous or even contradictory worldview. With the philosophy of Jim Jones, the facade of Christianity was used to attract followers to something that combined genuine Biblical egalitarianism of race and class with the pseudo-deification of Jones himself, among other things. However, if allegiance to a person or their proposed philosophical system makes an ideology cultish, then many religions are cults as they are.
Islam would be a cult because of its reverence for Mohammed, Mormonism because of its founder Joseph Smith, Catholicism for its adoration of Mary, and Christianity for its emphasis on Jesus--though, unlike many insist, Christianity is first and foremost [1] about things like ultimate truth, Yahweh as the uncaused cause, and moral obligations rather than mere Christological salvation from deserved annihilation in hell. However, the figures associated with these religions are either long dead or, if Christianity is true, have ascended to heaven in the case of Jesus (Acts 1). Their religions persist or thrive as active movements after their death. Famed religions are usually fairly organized in their theology, but the same could be true of a cult's theology.
Jesus himself would have had similarities to many more recent cult founders, such as appeal to the poor, a level of very controversial charisma, and the gradual acquisition of followers. This does not erase the evidence for the historical presence, death, and resurrection of Jesus, with the writings from the likes of Tacitus and Josephus pointing to the former two and the lack of a body pointing to the latter of the three. There is something far more important than whether or not there even is a difference between a religion and cult besides the exact philosophical stances each respective one entails. That is if any of them are actually true.
Since most religious tenets beyond the basic existence of an uncaused cause, the creation of the cosmos, and so on cannot actually be proven (as with scientific laws and their ongoing metaphysical uniformity) despite whatever evidence Christianity boasts, the most one could do is identify contradictions in miscellaneous worldviews that make them logically impossible because they would defy self-necessary axioms. This can reveal that some "cults" misrepresent ideologies like Christianity that they are claimed to endorse. It can also reveal that religions like Islam contradict their own tenets or affirm logically impossible things [2]. This does not mean there is a difference of any ultimate kind between a religion and a cult as opposed to different kinds of religions and cults.
The word cult is frequently reserved for fringe philosophies and movements associated with them. Perhaps used to promote fear or mockery, this word can be chosen out of emotionalistic panic or avoidable confusion about a system's real concepts. It is this often negative or dismissive connotation that leads some people to believe cults are different from religions for some arbitrary factor or something that would not even apply to all of what they would respectively call religions and cults. Like faith [3] or pornography [4], there is not one thing that might be meant in society by the term cult, and it is logically necessary truths about concepts that matter instead of petty linguistic constructs. No other core truth is ultimately altered because of this: all religions are cults depending on what is meant by the words. What is of significance is whether any religion is true.
[3]. https://thechristianrationalist.blogspot.com/2019/06/the-evangelical-misconception-of-john.html