What makes a Christian different from a non-christian?  

This is a question that ought not be answered too quickly.  

If we try to convince the non-christian world that we are smarter, free from 
moral impurity or always "good" then we not only make ourselves to be fools in 
thier sight for such an obvious fantasy but we also contradict the Scriptures 
which say, "If we say that we have no sin, we deceive ourselves, and the truth 
is not in us." (I John 1:8).  

What is it then that makes the believer different?  

It is not that he is without sin, but he knows what to do about his sin.  

He knows he must admit it.  

That is the literal meaning of the word (homologeo) translated "confess" in I 
Jn. 1:9 "If we confess our sins, he is faithful and just to forgive us our 
sins and to cleanse us from all unrighteousness."   

The proper orientation of the Christian is one of confession not of denial.  
If we are busy denying our sins and trying to convince others that we are 
"perfect" in practice then we waste our time and deny the very work of Christ.  

We must rather testify that it is by His death on the cross that we are 
forgiven.  He paid the debt we owe to God's justice.  It is HIS righteousness 
that we posess, not our own.  We then point others to Messiah, not to 
ourselves where they will find fault.  

So then our personal walk will also change.  Instead of covering our sins we 
quickly acknowledge them and come to Christ to have them forgiven and the 
desire for them removed.  So we strive to place all our trust in His work.  

We are "saved" by grace through faith, not by works lest we should become 
boastful (Ephesians 2:8-9).  

Jesus was the Lamb of God who came to bear the sins of His people.  

It is Him that we must glorify in all things.  

Christians are only sinners who have been saved by grace.