The Constructive Curmudgeon, or whatever he’s calling himself these days, has written a post on that question. His answer is clear, involved, and based on Scripture. (Thank goodness!) The first paragraph says:
A Christian is one whose identity and purpose are defined by being a believer in and follower of Jesus Christ. Since many cult members who hold heretical doctrines claim to believe in “Jesus Christ“ and to be “Christians,” it is paramount to remember that Apostolic Christianity was greatly concerned—as we should be as well—that people believe in the genuine Jesus, not some counterfeit Christ (2 Corinthians 11:1-4). The Apostle John stipulates that true and saving belief affirms that “Jesus is the Christ” (the divine Messiah) and that he “has come in the flesh,” which refers to his true humanity (1 John 2:22; 4:1-3). A Christian believes that Jesus is the Incarnate Word (John 1:1-3; 14, 18). Belief in Christ requires a monotheistic worldview. These Christological stipulations rule out any religious groups that deny the true deity of Christ (Colossian 2:9), such as non-Trinitarian Jehovah’s Witnesses or those teach that Jesus is one god among many (polytheistic Mormons) or those claiming that Jesus tapped into a universal “Christ Consciousness,” which is available to anyone sufficiently enlightened (New Age adherents), and so on. Christological errors are many (2 Corinthians 11:3-4), as is the wide path of destruction, as Jesus taught (Matthew 7:13-14).
Read the whole thing on his site.