By the end of the 12th century they had become holy days of obligation across Europe and involved such traditions as ringing church bells for the souls in purgatory. In addition, "it was customary for criers dressed in black to parade the streets, ringing a bell of mournful sound and calling on all good Christians to remember the poor souls." "Souling", the custom of baking and sharing soul cakes for all christened souls, has been suggested as the origin of trick-or-treating. The custom dates back at least as far as the 15th century and was found in parts of England, Flanders, Germany and Austria. Groups of poor people, often children, would go door-to-door during Allhallowtide, collecting soul cakes, in exchange for praying for the dead, especially the souls of the givers' friends and relatives. Soul cakes would also be offered for the souls themselves to eat, or the 'soulers' would act as their representatives. As with the Lenten tradition of hot cross buns, Allhallowtide soul cakes were often marked with a cross, indicating that they were baked as alms. Shakespeare mentions souling in his comedy "The Two Gentlemen of Verona" (1593). On the custom of wearing costumes, Christian minister Prince Sorie Conteh wrote: "It was traditionally believed that the souls of the departed wandered the earth until All Saints' Day, and All Hallows' Eve provided one last chance for the dead to gain vengeance on their enemies before moving to the next world. In order to avoid being recognized by any soul that might be seeking such vengeance, people would don masks or costumes to disguise their identities".