The nine marks of a cult (sources: Understanding the Cults by Josh McDowell & Don Stewart; The Four Major Cults by Anthony A. Hoekema):

1.  A source of authority other than, or in addition to, the Bible (including new truth or new interpretation of Scripture).

2.  Salvation by works, denial of justification by faith.

3.  Devaluation of the person of Christ (denying His deity or His humanity).

4.  Denial of the Trinity or an inadequate view of the Trinity.

5.  The group as the exclusive community of the saved.

6.  A strong, charismatic leader.

7.  False prophecy.

8.  Changing theology and subsequent practices.

9.  Deceptive double talk, saying one thing in public and another in private.