Christian is a label.  Biblically, you are either in a right relationship with God or you are not.  But even someone who makes a profession of faith in God and in Jesus work on the Cross makes mistakes.  In I John 1:9, we see God's requirement for dealing with sin as a believer.  Doing something bad is due to our sin nature and lack of self-control.  But doing something wrong doesn't mean we stop being a Christian.  My point to you has always been that everyone who goes by the label "Christian" isn't a biblical Christian.