So Barabbas is a significant person in the gospel account, chiefly because he is the living embodiment of a helpless, hopeless sinner who is spared from condemnation—even given an undeserved place of privilege—just because Christ took his place on the cross. Barabbas is a flesh-and-blood symbol of every redeemed sinner. In a true and literal sense, he could say "Christ died for [my] sins"—and he was no doubt the very first person to whom it might have occurred to make a confession like that. During those dark hours while the crucifixion drama was playing our, while the disciples were confused and scattered, while even those closest to Jesus wondered at the meaning of it all, Barabbas was already fully aware (in a unique and particular sense) that Jesus was dying in his place. I'm not suggesting he knew this with the full conviction of saving faith. But in a rudimentary sense, he must have had some crude understanding of the principle that lies at the heart of the atonement—because in a literal, physical sense, Christ had taken his place on the cross, borne the condemnation that was due Barabbas, and made it possible for Barabbas to go free—all without any work or merit on Barabbas's part. He did not deserve the favor he was shown.
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That is what the gospel is all about. The cross is the heart of the gospel message.