The way Saint Peter explains it to the Devil who's knocking on Heaven's door to collect his share of souls: there will be no more souls, as God has come to doubt if humans are really meant to achieve salvation. If they aren't then how can they be punished posthumous and sent to Hell? There's only one way to make sure if sinning is the human nature, or is it that they simply don't want to better themselves - Devil himself must go down to Earth, in human form, and if he can achieve salvation then so can a human being... Based on A. H. Tammsaare's classic novel of the same name.