I would say that a disciple of Christ is anybody who believes that Jesus is the Christ, wants to learn from Christ, wants to obey Christ. Does not matter what denomination, so he or she might be for example Catholic, or Orthodox, or Lutheran, or Baptist, or Trinitarian Pentecostal, or Oneness Pentecostal, or Jehovah’s Witness, or Seventh Day Adventist, or Mormon, or Living Church of God member, or Iglesia ni Cristo member, or Revival Centres Church member, or Amish, etc., or non-denominational, as long as he or she believes that Jesus is indeed the Christ, wants to learn from Christ, wants to obey Christ, then he or she is a disciple of Christ. I would say that this would not include for example Muslims or Baha’is, since they believe that some of Christ’s commandments are no longer valid for today, have been superseded by new commandments. So at most one could argue they want to obey Christ, only in terms as they believe Christ approves of commandments that he believes have superseded his own commandments.