In Christian circles we define a “cult” as a religious organization that calls itself “Christian,” but in reality is not Christian, because it does not believe the basic doctrine of the Bible that would make it Christian (i.e. being born again according to Romans 10:9). (see What is a Christian?)
Some examples of cults would be: the Jehovah Witnesses, the Church of Jesus Christ of Latter Day Saints (the Mormons), the Seventh Day Adventists, the Holy Spirit Association for the Unification of World Christianity (the Unification Church, the Moonies), Grace Communion International (Armstrongism, (formerly the Worldwide Church of God)) and all its splinter organizations, Christian Science, and Roman Catholicism.
However, it is possible that a person who is a Christian (i.e. a person who has been born again according to Romans 10:9) could be a part of an organization known as a cult. Therefore, we CANNOT make the statement that every single person who is a member of a cult is NOT a Christian. A person could have been born again before or after joining a cult. And even though they SHOULD NOT be a part of an organization like this, it doesn't change the fact that the individual is a Christian if he is born again.
Religious organizations that do not claim to be Christian are just simply “other religions.” Some examples of these would be Buddhism, Confucianism, Hinduism, Islam (Islamism, Muhammadism; the Muslims or Moslems), Judaism, and Shintoism.
Sometimes Christians say, “Christianity is not a religion. It is a relationship.” When Christians say this, they are emphasizing the fact that we have an actual relationship with God—that we talk to Him and He talks to us. We don't just have a set of rules to follow. However, technically speaking, Christianity is a religion. But it is the only religion that worships the true and living God that created us all. It is also the only religion that points the way to Heaven. You cannot get to Heaven by believing in any other religions or cults.