Milan is not a good example of a republic and should not be classified in the same position as Venice (Genoa or Pisa are much better examples) therefore the question is not entirely correct. Also majority of Central Europe was christianized mostly by missionaries from the Byzantine or the Holy Roman empires (with the exception of Great Moravia, who indeed started the process of Christianization trough Irish missionaries sent by the bishop of Salzburg).
Sorry for the mistake and thank you for the comment. I replaced Milan with Genoa. I had also changed Central European to German before but had forgot to resubmit.