The above question can be taken in more than one way. One answer would be that it means to be acceptable to God, to be safe from God’s wrath, to be a part of an elite group which enjoys God’s exclusive approval and blessing. Being a Christian thus means that one is very special in the eyes of God.
Of course, the other aspect of this question is one of how one gains the identity of Christian and thereby gains the associated privileges. This latter question has never been settled once and for all.
On the macro level, the associated question is this: What does it mean to be a Christian nation? In one sense it simply means being a part of western civilization where Christianity by its various measures has been the predominant religion for centuries. Some want to identify a Christian nation as one which is based on Christian principles; but, since those principles are much debated and even subject to constant review and revision, this definition is no more concrete than that of the individual Christian.
One thing that most would probably confirm is that to be a Christian means to be an avid warrior against the forces of evil. By extension then, those who take up arms against evil, as defined by the church or the state, become de facto Christians. If, when you violently confront and destroy the forces of evil, you are doing God’s work, it seems to make sense that you should thereby gain God’s approval, which is the same as being a Christian. This sort of reasoning would appear to explain how and why the institutional Christian church can remain silent about the implications of engaging in state sponsored combat as a non-Christian. The glory of the warrior role can only be maintained if all dead soldiers are Christian and therefore safe from eternal torment as normally prescribed for non-believers.
So what if anything can we conclude about the initial question? That is completely a personal issue for each of us. For me, it means that the definition of a Christian is often more a matter of politics and geography than it is of theology.