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 â€œJesus Christâ€œ and to be â€œChristians,â€ it is paramount to remember that Apostolic Christianity was greatly concernedâ€”as we should be as wellâ€”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 â€œJesus is the Christâ€ (the divine Messiah) and that he â€œhas come in the flesh,â€ 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 Jehovahâ€™s Witnesses or those teach that Jesus is one god among many (polytheistic Mormons) or those claiming that Jesus tapped into a universal â€œChrist Consciousness,â€ 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.