After having repeatedly examined my conscience before God, I have come to the certainty that my strengths, due to an advanced age, are no longer suited to an adequate exercise of the Petrine ministry.
"For this reason, and well aware of the seriousness of this act, with full freedom I declare that I renounce the ministry of bishop of Rome, successor of Saint Peter, entrusted to me by the cardinals on 19 April 2005, in such a way, that as from 28 February 2013, at 20:00 hours, the See of Rome, the See of Saint Peter, will be vacant and a conclave to elect the new supreme pontiff will have to be convoked by those whose competence it is.
That's quite a resignation letter.
I expect he's realised that god is entirely fictional - I can see how that would dent one's enthusiasm for the job.