Traditionally, the word "Christian" referred to anyone who identified with any part of the Christian faith, regardess of their denomination, the details of their beliefs, or their levels of practice. Thus, anyone who identified him/herself broadly as part of the religion which followed the life and the teachings of Jesus was a Christian. Simple.
Lately, it's become a bit different ... and a bit more complicated.
Some Christians have started to say things like this about other Christians because of various details of their beliefs or practice: "This person isn't a real Christian," or "The people in that church aren't real Christians".
It's got to the point in the United States where the media uses the word "Christian" only to speak about the more narrow variety of "Protestants". Increasingly, the media here in Australia is starting to do the same thing.
This situation isn't helped by the fact that many of the groups that identify themselves most deliberately as "Christian" tend to represent a particularly narrow approach to Christianity.
For example, there's a group called the Australian Christian Lobby They represent the views of a particular group of Christians, but not of all Christians. The Christians whom they represent are those with particularly conservative religious beliefs (or, at least, conservative beliefs within a "Protestant" context), particularly conservative political opinions, and particularly conservative views on sex. (And they spend a lot of energy expressing their views on sex.) They have a perfectly legitimate right to hold and promote these views, but it is dishonest for them to act as if these are the only possible views that can be held by Christians.
As well, it is foolish for the media - and for politicians - to think that their views are the sum total of Christian opinion in Australia.
And it's particularly foolish for anyone to think that Christians who hold contrasting opinions are somehow being other than Christian in their views.
Anyway, in the context of all of this, here's my working definition of the word "Christian":
The word "Christian" refers to any individual, faith community, or movement for whom Jesus of Nazareth is the central figure in their spirituality; without regard to whether or not other Christian individuals, communities, or movements regard their beliefs and practices as being adequate.
This definition has the advantage of regarding as a Christian any person who relates in a positive way to the Christian faith, without imposing a "Christian" identity on those who do not choose such an identity.