21 God made him who had no sin to be sin for us, so that in him we might become the righteousness of God.
"Christ becoming sin for us I believe refers to his incarnation, coming in the flesh. In the New Testament the word "flesh" is "sarx" in the Greek and is the same word the NIV translates as "sinful nature" Look at this video.
That video really sums up what the Bible verse is saying. WE are forgiven because God sent his son Jesus, who had never sinned, to die upon the cross to save us, who sin every day. Isn't that just wonderful?