In what way did Jesus actually 'become that' on our behalf?
He who knew no sin became sin for us, taking the place of a rebellious, wicked son under the curse. Though personally sinless, He was crucified and hung on a tree, and God laid on Him the sins of the whole human race, so His death was a curse, not something beautiful.
Les explains that Peter was bringing the Deuteronomy 5 picture to Israel: a rebellious, drunkard, wicked son was stoned and then hung on a pole as a public spectacle in extreme situations. Peter was showing the nation that this is what they had really done with their Christ—they considered Him like such a rebellious son, killed Him, and hung Him on a tree. In actuality, though, He became that on our behalf. He who knew no sin became sin for us. Les also stresses that the crucifixion was awful, not beautiful; it has been glamorized with jewelry and golden crosses, but it was a curse, because God laid on Him the sins of the whole human race.