Christianity is being a follower of Jesus Christ.  That means a Christian is someone who knows Jesus and is trying to be like him.  However, those who seriously attempt to be like Jesus discover that such a thing is impossible.  We all fall far short of the standard Jesus proclaimed and demonstrated.  In the Bible such shortcomings are called sin.  Sin causes us to be separated from God, and to deserve the punishment of death – spiritual death.  But Jesus not only lived a perfect life, He also died for those who can’t live up to his standard.  Jesus paid the punishment we deserve when He died, so now we can be brought into a relationship with God and empowered by God to live like Jesus.  “Christians” are those who have such a transforming relationship with God.  They have completely trusted Jesus’ death as a substitute their own and have given their lives entirely to Him as their Master.