What is the Biblical definition of what makes one a Christian? Many religious leaders/organizations place the "cult" label on groups of people or churches who interpret the Bible differently than them. If we claim the Bible as our only authoritative source for our doctrinal foundation. What does it actually say on the matter?
The disciples were called Christians first at Antioch. (Acts 11:26, NIV) 

Therefore, a Christian is a "disciple" of Christ. And a "disciple" of Christ is one who "believes Jesus and abide in his word" (John 8:31).

Therefore, one who does not believe Jesus or does not abide in the word of Jesus is not a "disciple" of Christian and not a Christian.