Well, let us remember that there are only five characters in this excellent short piece of writing: the sinner who dies, the accuser, Peter, David and John. The sinner who dies we are told lived a life of sin for his seventy years until just before his death, when he confessed and turned to God. It is he who goes to heaven and tries to gain admittance through the pearly gates.
The accuser is the one who recounts all of the sinner's many misdeeds and failings throughout his life, which threatens to disqualify him from entering heaven.
Both Peter and David play the role of doormen to the gates of heaven. They hear the sins of the man and say that he cannot enter, but are silenced by the sinner's comments regarding their own sinfulness and failings during there life.
Finally, the Apostle John is the man who gives the sinner admittance to heaven for his belief about love and how "God is love." John was therefore unable to drive the sinner away, as the sinner realises, and therefore opens the gates to heaven for him.