I understand that during the Middle Ages, if a king changed his religion the peasants had to as well. If a king converted to a circumcising religion, did that mean that there were then lines of men forced to get circumcised? I have a hard time believing that peasants would passively just agree to this without a revolt. I know plenty of kings converted to Islam and I believe one converted to Judaism as well.
I hope this isn't a stupid question, I was just thinking about this the other day.
I know plenty of kings converted to Islam and I believe one converted to Judaism as well.
I assume by "Middle Ages" you were thinking "Middle Age Europe"? If so, I don't know of any European kings who converted to Judaism, and I don't know of any who converted to Islam, either. Outside Europe, you may be thinking of the Khazars. They converted to Judaism, but exactly what percentage of Khazar society converted is conflicted. As a consequence, there's no particular record of people actually being forced to convert (which would be outside the bounds of normative Judaism anyway), and certainly not evidence of objections to that requirement.
source: Golden, Peter Benjamin. "The Conversion of the Khazars to Judaism." The World of the Khazars. New Perspectives, Handbuch der Orientalistik 8 (2007): 123-62.