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This tale from the life of St Peter is recorded in the collection of legends written down by Jacobus a Voragine in the 13th century. It tells how the apostle, having triumphed over Simon Magus, was persuaded by the Christians of Rome to leave town. Jacobus a Voragine relates how Peter encountered Christ on the Appian way and asked "Quo vadis domine" (Whither goest thou, master?), to which Christ replied "To Rome, to be crucified anew."
This apocryphal legend is in fact the beginning of Peter's own martyrdom. This would certainly explain the vigorous movements in Carracci's painting, with the apostle recoiling in terror. It is not the unexpected encounter with the risen Christ that has taken the apostle aback, but his awareness of his own human frailty. Annibale's magnificent rhetoric reminds the spectator of Christ's call to turn back.
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Thanks for posting the story of that legend!