The Gospel by John MacArthur
"He [the Father] made him [the Son] to be sin who knew no sin, so that in him we might become the righteousness of God" (2 Cor. 5:21).
Jesus was guilty of nothing. Yet on the cross, the Father treated Him as if He had committed personally every sin ever committed by every individual who would ever believe. Though He was blameless, He faced the full fury of God's wrath, enduring the penalty of sin on behalf of those He came to save. In this way, the sinless Son of God became the perfect substitute for the sinful sons of men. As a result of Christ's sacrifice, the elect become the righteousness of God in Him. In the same way that the Father treated the Son as a sinner, even though the Son was sinless, the Father now treats believers as righteous, even though they were unrighteous. Jesus exchanged His life for sinners in order to fulfill the elective plan of God. And He did it so that, in the end, He might give back to the Father the love gift that the Father gave to Him.