British pastor and theologian, John R.W. Stott wrote in his book, The Cross of Christ, about what happens when we are united with Christ:
When we are united to Christ a mysterious exchange takes place: he took our curse, so that we may receive his blessing; he became sin with our sin, so that we may become righteous with his righteousness. . . . On the one hand, God declined to ‘impute’ our sins to us, or ‘count’ them against us, with the implication that he imputed them to Christ instead. On the other, God has imputed Christ’s righteousness to us. . . We ourselves have done nothing of what is imputed to us, nor Christ anything of what is imputed to him. . . . He voluntarily accepted liability for our sins, (pg. 148-149).
“For our sake he made him to be sin who knew no sin, so that in him we might become the righteousness of God,” (2 Corinthians 5:21, ESV).
HT: Ray Ortlund