The RCC pretty much made sure, there was no opposition to their rule, so it makes sense that reformers didn't come from a non-denominational church.
Apparently, the Reformers aren't exactly choir boys, but it would be fair to say, they had much different lives, than we have today in the Western World. These people risk their lives for standing up for their faith, that opposed the Catholic Church.
"It is true that Calvin and his fellow pastors in Geneva were involved in the death of Servetus. However, it would be difficult to find any church leader in the 16th century who advocated a more gentle approach. Luther called for attacks on German peasants and wrote an angry tract against the Jews, called ‘On the Jews and their Lies'. Zwingli, the Reformer of Zurich, supported the execution by drowning of the Anabaptist leader, Felix Manz. Sir Thomas More, England 's Catholic Lord Chancellor, presided over the execution of those he viewed as “heretics” in England during the reign of Henry the VIII. Each country of Europe in the sixteenth century felt that defending its religious views involved taking strong measures against those who disagreed. Toleration and acceptance of doctrinal differences were simply not sixteenth century concepts."
https://calvin.edu/centers-institutes/meeter-center/files/resources-page/TheServetusControversy.pdf
Peter denied Christ three times, Tom doubted, Judas betrayed Jesus, and Paul murdered Christians. Maybe the Reformers had a little Saul in them?!?!