I would argue that the central message of this fascinating piece of short fiction is the supremacy and power of grace. Let us just remember that before the story begins, Tolstoy adds the quote from the Bible refering to the sinner who dies on the cross next to Jesus and repents, and how Jesus says to him that, even though he is repenting just before death, he will be with Jesus that day with him in his kingdom. As the story begins, we realises we are being given another example of the supremacy of grace, as the description of the man who dies makes clear:
There was once a man who lived for seventy years in the world, and lived in sin all that time. He fell ill but even then did not repent. Only at the last moment, as he was dying, he wept and said:
'Lord! forgive me, as Thou forgavest the thief upon the cross.'
And as he said these words, his soul left his body. And the soul of the sinner, feeling love towards God and faith in His mercy, went to the gates of heaven and knocked, praying to be let into the heavenly kingdom.
The parallels between the two men are obvious. Both live a life of sin and turn to God in the last few moments of their life, trusting in God's overwhelming grace to be sufficient to more than make up for all of their sins. As the sinner speaks with first Peter and then David, he forces these figures to recognise the way that grace has played a vital role in their own journey to heaven, and thus is able to finally gain admittance into heaven thanks to the Apostle John and his creed of love. The theme of this short story is definitely that grace conquers all, even the most darkest and blackest sin, as even heroes of the faith, such as David and Peter, are shown to be just as dependent on grace as sinners like you and me.