A much more thorough answer to this question will be provided on the pages that follow, but in summary, the nutshell is this: There is enough diversity of thought on secondary issues of theology that people who feel strongly about these issues have difficulty being in close practical and functional unity with others who disagree with them. They have sorted themselves into multitudes of diverse organizations. Every movement, when it gets big enough, experiences this phenomenon. While there is fundamental agreement and unity on foundational issues, there is enough force of preference on smaller questions that it is sometimes impossible to keep functioning as one organization.
Every true Christian will agree to a set of foundational beliefs, codified in a small collection of documents called "creeds." These creeds lay out what the Bible clearly teaches on issues like the Trinity and the work and nature of Christ. If someone rejects these foundational teachings, they are not a Christian.