There are three crosses in Calvary. On one side of the cross is a man who died in sin - he did not accept Jesus. On the other side of the cross is a man who will die of sin - he believes that Jesus is the Savior and the Lord. On the cross in the middle is a man who died for sin - Jesus. He can die for others because he is the son of God and he has no sin of his own. For the two people hanging on the cross on both sides, the cross in the middle determines their opposite "destination" - eternal hell, eternal Heaven.
The two thieves and their "response" to Jesus represent the whole world. In these three crucified people, I see a picture of a sinner, a saint, and a savior.