"He [God] hath made Him [Jesus] to be sin for us, who knew no sin; that we might be made the righteousness of God in Him" (2 Corinthians 5:21).
This is beyond belief! An innocent man is about to be crushed by God. We have grown accustomed to seeing courtroom scenes on the news, but can you imagine seeing the lawyer on trial for the crime his client committed? Or even more incredible, can you imagine that the death penalty is handed out and it is the attorney who dies and not the defendant? That would be bizarre, but that's what we see when Jesus, our sinless Advocate, receives the punishment we—who are guilty—deserve.
Jesus is our High Priest, but what kind of priest is this who becomes the sacrifice? Priests offer sacrifices—but this priest is the sacrifice. This priest lays Himself on the altar. You see, Jesus died for sin—but not for His own sin. He had no sin. He was in every sense made sin for us. He became all of our rebellion, all of our lying, all of our cheating, all of our adultery, all of our filth, all of our ugliness. He became all of that on the cross. Otherwise, how could God crucify His Son?
Here's the gospel in a phrase. Because Christ died for us, those who trust in Him may know that their guilt has been pardoned once and for all. What will we have to say before the bar of God's judgment? Only one thing. Christ died in my place. That's the gospel.
—Condensed from "Jesus Our Substitute" by Alistair Begg.