Union with Christ The Imputation of Sin and Righteousness

2 Corinthians 5:21   For He made Him who knew no sin to be sin for us, that we might become the righteousness of God in Him.

Christ was made to be sin.  Well, actually he remained righteous.  He was reckoned to be sin.  If he weren't righteous when he was reckoned to be sin, then he could not have been our sacrifice for sin.  Therefore, he must remain perfect all the time he is made sin.  This is the imputation of our sins to Christ.

The reason for this reckoning Christ to be sin is that, as a consequence of being in him, we might become the righteousness of God.  It seems most likely that the intent of the writer is to say that we become righteous in reciprocal manner to the way that Christ became sin.  He remained righteous, while he became sin.  Likewise, we remained sinners while becoming the righteousness of God.  All this happens in Christ, that is, in and because of our union with Christ.

Consequences:

Union with Christ does not mean that he and we are morally mixed, such that he becomes partly a sinner and we become partly righteous.  He is reckoned to be sin, though he is not a sinner at all.  We are consequentially reckoned to be righteous, though we are not righteous at all.  Christ is not made sin, such that he becomes partly sinful.  We are not reckoned to be righteous because we are made partly righteous by our union with Christ.

Therefore, it is right to say that union with Christ does not dispense with the need for imputation.

But, there's more:

Forensic relationships are the necessary foundation for the promotion of actual righteousness.  Christ was reckoned to be a sinner forensically, but he really grasped both the guilt and power of our sin, and confirmed both the judicial basis of our acceptation as sinners, and the basis in divine power for our renovation in righteousness.  Likewise, though we were reckoned forensically to be the righteousness of God, our forensic justification as sinners must ultimately eventuate in real righteousness, because the sacrifice of Christ takes away both the guilt and power of sin.  In the end, we will be like him, for we shall see him as he is.

But, we cannot lose our connection with Christ in the righteousness of our own resurrection, so that the imputation of Christ's righteousness is forgotten.  We never become autonomous from him, but remain eternally dependent on our union.  We will recognize forever that the reward of our final destiny is simply Christ's crowning of his own works in us, which were done by him when we were very imperfect.  Even in the beauty of holiness, we can only attribute our glory to his gift!