During Europe in the Middle Ages, how religious was the "average Joe"? Looking at the large scale politics of the time it seems religion was a major part of the elites' lives, with treaties with the papacy, crusades, excommunications of kings etc. But to my knowledge all this is geared around the upper classes - knights, lords, barons and kings, and was to a large extent realpolitik. Do we have any evidence that peasants were either insincere in their profession of belief (a profession which was compulsory, I'm pretty sure), or at least any lack of interest or ambivalence in organized religion?
I'm inclined to turn around the assumptions in your question re the powerful vs. the powerless:
IMO the dual powers of popes and emperors did not prevail mainly for spiritual reasons. They were the same stuff as "realpolitik" is still made of: perhaps think of excommunications as U.N. mandates of their days :)
On the other hand, I could imagine that to "average Joes" religion was dearer than to the powerful in their official functions. Why? There was no psychological branch of medicine nor self-help book industry around. I've recently heard that (I think) 26 percent of Americans suffer from some form of diagnosed/treated depression. Now we live in demanding times, but so did folks during the Middle Ages. I am inclined to think that religious institutions served (mainly or also) somewhat vital "worldly" roles then, roles that we tend to underestimate from today's perspective of relatively secure medical care, more enlightened understanding, etc.
So how religious was the "average Joe": well, perhaps (also) "26 percent", or in any case more than we perhaps allow today.
I answer pretty much every question here with 'it depends'. This one is no exception: It depends what you mean by 'religious'.
Did the 'average person' believe in a supernatural being? Yes ... 'non-belief' or atheism simply do not exist in the European middle ages. Simply put - absolutely everyone was religious, if you take 'believing in God (or perhaps gods)' as the qualifier.
However if you take 'religious' to mean something more specific - such as regular church-going, or adherence to the strict tenets of the medieval Church, the answer would be very different. At the peasant level, it's pretty well established that Christianity could mix with all manner of other beliefs and superstitions to create something that was far from standard religiosity.
Moreover, the Church did not set great store by attendance at church by the general population. A peasant might be expected to attend Church at Easter or Christmas, but the rest of the year was pretty much optional. (Paying your tithes - not optional, of course).
The main 'business' of the church was not seen as providing a service to the rank and file population, but instead it was the carrying out of masses and prayer in the great monasteries, priories and cathedrals. The religious well-being of the individual was far subsidiary to the interests of the great churches, and parishes and parish clergy quickly came to be primarily tools by which money was syphoned upwards to the canons, prebendaries, deans, archdeacons, priors, bishops and abbots. Saying so is not to criticize the pre-Reformation Church, but rather to simply point out that it was in those offices that society viewed the primary concerns of the church to lie. It is only later generations that have come to see the religious care of the individual as the church's 'main job'.
So another way to answer your question would be to say - everybody was religious, but the majority of people were perhaps hardly more regular churchgoers (or more interested in the Church) than a modern agnostic.
Gorski 2000 seems to indicate part of the problem is the fetishisation of organised religion as an indicator of peasant religiosity and the corresponding privileging of Christian narratives of appropriate religious behaviour in medieval Europe.(Van Engen 1986 in Gorski 2000) The peasantry depicted in all these historiographies was fundamentally religious—what it was not what organised or Christian in comparison to the opinion of the Church. So the answer is "yes, they were fundamentally religious, but your question is wrong as it incorporates unacceptable normative assumptions." Medieval popular cultures were local, Christian, Magical, Pagan, Churched and non-Churched. But fundamentally religious in the sense that a metaphysical determination of reality was constant, and the observation or propitiation of a reality other than the apparent was required.
Gorski supplies a full literature review with critical analysis as of 2000. Any question you may have on the topic will be fully covered in Gorski.