It is fascinating the many debates that can and have been had over the true meaning of a collection of texts written 2,000+ years ago. And most people are sincere, and can point to chapter and verse why they believe the way they do (obligatory cartoon included below). "Cult" often just means "a small group that we don't agree with and don't want to lose members to" (you wouldn't go near a cult, would you?) At least nowadays disputes between different groups are generally not settled with the sword, as they were from the time of Augustine on.
Once a group is founded principally on seeking truth rather than love I suspect division becomes almost inevitable. With great sincerity different parts of the group come to different conclusions on minor beliefs, and neither side can back down since they alone have the truth. So they metaphorically (or literally) consign the other side to hell and become two even smaller and more insular group. I think the exclusivity talked about is actually needed to maintain the fiction mentally that they alone have the truth and all the other sincere seekers are wrong.
As for the Trinity, it's weird: I literally heard it said recently that all the Trinity analogies (three leaf clover, for example) are heresies and should only be used to teach you what the Trinity isn't. Basically, that it is a spiritual truth which is so great that it cannot be represented by any human analogy. But important parts of it can and have been demonstrated from scripture and it is the position mainstream churches have upheld for most of the history of the church. Whether or not they can be called cults, groups that reject it can reasonably be called unorthodox.