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The three ecclesiastic electorates; Trier, Cologne, and Mainz have well documented lists of the Archbishop-Electors who served and at what times. The majority of them in the latter period of the Holy Roman Empire are indeed of noble descent.
However, John I of Trier is presumably to have been of commoner descent, as stated in this 1881 publication by Bavarian Academy of Science.
John I (German: Johann I.) (c. 1140 - 15 July 1212) was the Archbishop of Trier from 1189 until 1212.
From what little is known, John originated from the region around Speyer. In 1173 he appears listed as an archdeacon of Speyer and the provost of St German-Stiftes. From 1186 until 1189 he was the chancellor of the Holy Roman Emperor Frederick I Barbarossa. In September 1189 he was appointed the Archbishop of Trier. Bishop John increased the possessions of the archdiocese during his time by gaining several countships and castles.
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