II Corinthians 5.21

Our Lord Jesus Christ was 'made sin' or 'became sin' for us, because He assumed our sin in His own soul and body on the Cross for our redemption: 'who Himself bore our sins in His own body on the Tree, that we, having died to sins, might live for righteousness' (I St Peter 2.24). On the Cross, there is a divine exchange in which Our Lord willingly takes on Himself our sin in satisfactory and real atonement, offering His life for our sake, that we may be joined to Him and in Him in his perfect and all-sufficient act of obedience, worship and love rendered to the Father: in turn, through Christ our Priest and Mediator, we are given the righteousness of Christ as real gift, which is not merely imputed to us for Christ's sake, but is imparted and infused in us by divine grace, especially through the Sacraments of Baptism and Holy Communion.

Christ, the immaculate and all-sinless One (Hebrews 4.15), the Lamb without spot or blemish (I St Peter 1.19), offers Himself for us and offers us in Himself to His Father in self-oblation, the unique self-donation of His perfect will united in love to the Father's will. Through Christ we have access to the Father through the one Spirit (Ephesians 2.18).

We 'put on Christ' as God's own purchased possession, His children, in Baptism (Galatians 3) and Holy Communion (I Corinthians 10.16-17) and made one with Him, that He may dwell in us and we in Him (St John 6.53-58). The Person and Work, and the merits and atonement, of Our Lord are conveyed and applied to us through the Sacramental System of the Church. In Christ, we become the righteousness of God, that is, we are made right with and acceptable to God through the merits and perfect righteousness of Our Lord. Our justification in Christ is a free gift of unearned and undeserved grace, the infused energy of the Spirit, whereby we become partakers of the divine nature (II St Peter 1.4), partakers of the life and communion of the Holy Trinity, and are adopted as children of God and sons of grace through the Holy Spirit and the divine sonship of Jesus Christ (Romans 8.1-17). We become filii in Filio, sons in the Son. We become the Body of Christ (I Corinthians 12) through union with Christ's Body - offered on the Cross, glorified in the Resurrection and Ascension, present in the Sacraments, and extended in the Church.

The Orthodox Study Bible puts it this way:

How was Christ made to be sin for us? He, the incarnate Son of God, voluntarily assumed the consequences of our sin — corruption and death — without sinning Himself. And He submitted to unjust suffering because of the sinful passions of men and of angels. This means salvation is far more than forgiveness of sins. It is new life: our reconciliation to God (vv. 18-20) and our becoming new creatures (v. 17), participants in the very righteousness of God (v. 21). This means our salvation is not just juridical, (the static, legal pronouncement of a judge), but personal and relational (the dynamic, sacrificial love of a father for his child).