See The Kingdom of the Cults by Walter Martin. Here are the definitions in that book:Biblically what constitutes a cult?
"...In his study of modern American cults and minority religious movements as found in his text These Also Believe, Dr. Charles Braden, emeritus professor at Northwestern University (1954) [and co-author, John C. Schaffer, lecturer (1955) and visiting professor at Scripps College (1954 to 1956)], made a number of observations with which this writer agrees. In regard to the term “cult,” for instance, Dr. Braden says the following: By the term cult I mean nothing derogatory to any group so classified. A cult, as I define it, is any religious group which differs significantly in one or more respects as to belief or practice from those religious groups which are regarded as the normative expressions of religion in our total culture (Preface, xii).
I may add to this that a cult might also be defined as a group of people gathered about a specific person or person’s misinterpretation of the Bible. For example, Jehovah’s Witnesses are, for the most part, followers of the interpretations of Charles T. Russell and J. F. Rutherford..."
For Christians, probably the best way to identify a cult is to determine whether they reject or modify the doctrines of the deity of Christ (that Jesus is God) and the Holy Trinity (or triune Godhead). Those who attack the doctrine of Christ are called "antichrists" by the apostle John.