What does it mean to be a Christian? The trouble with asking this question is due to the theological/interpretive world that Protestantism/American evangelicalism has created. Because of the rebellious foundations of Protestantism, the answer to this question has become one of private interpretation or preference. Because the evangelical world operates on the premise of, “What does this verse/passage mean to you?”, there is no final authority to which to appeal.
If anyone has been visited by door-to-door missionaries, you will understand that debating over the meaning of biblical passages is futile, for personal interpretation is the foundation for evangelical belief. To ask the original question, one will get varying answers from the multitude of denominations or groups.
While some answers may be similar, the definition of words can be very different depending on the denomination. This again reduces to the question of authority. For example, the Methodist denomination broke away from the Anglican church, which broke away from the Roman Catholic Church. But being a splinter of a splinter does not make one’s theological premises necessarily correct.
To see the error of one’s predecessor is a good thing but to merely splinter away based on one’s private interpretation is to commit the same fallacy as the original splinter. Instead of looking at Rome and critiquing their theological views as aberrant when compared to one’s own private views is faulty.
As the Nicene Creed says, “There is one, holy, apostolic Church” and this is the standard by which all theological belief and practice should be analyzed. To use the metaphor of mathematics, unless one has the 90-degree standard by which to judge all angles, the end result will not be the right angle.