Christianity tends to put more importance on proper belief than many other religions. When you say that someone is a Christian, you normally mean that he accepts the major Christian beliefs. That's not the whole story, since Christianity is also a way of life and a community. But most Christians do not think it is appropriate to apply the term Christian based simply on the fact that someone has Christian parents and grew up as a Christian, or even based on the fact that they admire many of Jesus' teachings. To be classified as a Christian, one is normally expected to accept the major Christian beliefs, to be following the way of life that Jesus taught, and to be a part of the Christian community.
Most Christian groups have standards of belief.
Members are expected to accept the standards of their community. This is not to say that Christians have no questions or doubts. However most groups expect leaders and teachers to advocate orthodox positions. Groups differ both in the way their standards are codified, and in the degree of conformity that they expect. Some have detailed formal standards of belief.