There are three views of what defines a Christian:
First is a general definition that says anyone living in a “Christian” country is a Christian. A Christian country would be defined as a country where Christianity has historically been the de jure or state religion. Thus anyone living in a Christian country would by definition be a Christian. This is the kind of Christian the Muslims were attacking when they destroyed the World Trade Center in 2001. It is mostly related to where a person lives and the kind of air, in general, they breathe.
The second is that a Christian is someone who believes in the sacrificial of Jesus Christ and who, when they die, will go to heaven. The emphasis is on personal and individual salvation, based on his profession of faith in Christ. There is usually an assumption of a changed life as the Spirit of God lives in the Christian, but not this is not always the case. This definition of Christian has its source in what the believer does with respect to what God has done in history.
The third is that a Christian is someone who professes faith in the sacrificial of Jesus and who has been baptized into the Name of Jesus (including their baptized household). In this definition of Christian there is no particular claim to individual salvation other than that being a Christian means being a member of the covenant community which is Christ’s body, the church. Because the church is Christ’s body, however, individual salvation is assumed because of membership and a life of faith rather than because of a “commitment” or a declaration of faith as in version two above. This definition of Christian has it’s roots in what God has done for the believer in history and in the life of the believer directly.