What faith were almost all of the people during the Middle Ages?

I assume that this question refers to the people of Western Europe.  If so, the correct answer is that almost everyone in Western Europe during the Middle Ages was Christian.  More specifically, almost everyone in Western Europe during the Middle Ages belonged to the Roman Catholic Church.

Because the Roman Empire covered most of Western Europe, and because Christianity was not persecuted after Constantine, the faith was able to spread over most of Europe.  Christian...

I assume that this question refers to the people of Western Europe.  If so, the correct answer is that almost everyone in Western Europe during the Middle Ages was Christian.  More specifically, almost everyone in Western Europe during the Middle Ages belonged to the Roman Catholic Church.

Because the Roman Empire covered most of Western Europe, and because Christianity was not persecuted after Constantine, the faith was able to spread over most of Europe.  Christian missionaries spread out across the region and brought the faith to places throughout the empire and (like Ireland) beyond.  In this way, practically everyone in Western Europe became Christian.  People in almost every corner of the region had abandoned their previous, pagan, beliefs by this time.  There were some Jews in most European countries, but not many.  Before the Reconquista, there were Muslims in Spain, but not really in other parts of Western Europe.  This meant that the vast majority of people in Western Europe were Christian.

At that time, to be Christian in Western Europe meant that you were Roman Catholic.  The Protestant Reformation had not yet occurred, so the Christian Church was still unified in Western Europe (in the east, most people were Orthodox).  Therefore, the best answer to your question is that almost everyone (assuming we are talking about Western Europe) was Roman Catholic during the Middle Ages.