What religion are medieval guilds?

Answer 1

Everyone was Catholic in the middle ages.

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We can probably safely assume most guild members were Catholic. Guild charters would have been likely to exclude Jews, and there were not enough Muslims in European areas with guilds to be likely to be guild members. That said, we can be sure that a number of guilds in various towns and cities were dominated by members of sects the Catholic Church regarded as heretical. This is probably particularly true of Hussites and Lollards, who were numerous in the Late Middle Ages when the guilds were operating.