2 Cor. 5:21" *..he hath made him to be sin for us,who knew no SIN;that we might be made the RIGHTEOUSNESS of God in him."*

This is the doctrine of substitution.

He became you,that you might become Him.

Little wonder,2Co.6:14-16,refers to the new creation,as:CHRIST,LIGHT, RIGHTEOUSNESS.

"..what concord hath CHRIST with Belial?or what part hath HE THAT BELIEVETH with an infidel?

If you're born again,you are called Christ,because you're now his offspring-a product of the second Adam,born not in the class of men,but in the class of God.

The word,Christ,is an office,occupied by Jesus,who is the Christ.
Jesus,is His name.Christ is His office;which means:the anointed one,and his anointing.

In Christ,you have become a partaker,of that exalted office,by virtue of the fact,that you have the same nature with Jesus Christ

This is the reason,2Co.6:15,calls you- Christ.

Meaning-you are an extension of the Messiah on the earth,TO SAVE MEN.