I know two major challenges to official dogma (at least in Catholic Europe) arose with the Protestant Reformation and the secular Enlightenment, both after the Renaissance. Even before that, heretical sects arose like the Hussites and Cathars in the West and the Paulicians and Bogomils in the East. But most of those movements were rooted in alternate interpretations of the Bible and Christianity, as was the Protestant Reformation later on. Did anyone from medieval Christian communities actively question Christianity itself?

In medieval Western Europe, neither church nor state had enough organised power to enforce perfect orthodoxy at the time. For example, various folk beliefs hung around from before Christianity, and the Catholic Church didn't seriously try to crack down on those until it got reinvigorated after the Renaissance. Moreover, Catholicism has never emphasised Biblical literalism the way some Protestant denominations do. So I figure that less strict believers didn't get in serious trouble unless they publicly challenged the Church. Again, this mainly applies to Western Europe; if anyone knows about the situation under the Eastern Church, I'd greatly appreciate it.

But my question is if anyone in medieval Christendom questioned the validity of Christianity and the Bible. Obviously, your average peasant would lack the education to know all that much about what he was supposed to believe, but beyond that, is there evidence that people actively doubted the religion itself? Of course, they probably kept those doubts to themselves, and they probably didn't have enough scientific knowledge to abandon belief in the basic concept of God. But you can believe in God without avowing the dogma of an organised faith like Christianity.

On a similar note, did anyone raised Christian convert to another religion? If that did happen, I'd assume it happened within Christian communities in close proximity to Jewish and Islamic communities, or in places and eras where paganism was still ticking around.