It's surprisingly hard to say. The big problem is that even near the end of the Middle Ages literacy was maybe 10% and maybe as low as 5%. So most of the voices that we encounter with respect to religious life in the Middle Ages are going to be clerical. What do clerical voices tell us?

Every medieval European -- aside from Jews and Muslims in newly-conquered regions of Spain and Italy (or Muslim merchants in port cities) -- was assumed to be Christian. That means that they're baptized as a baby and, by the twelfth century, assumed to have at least a baseline knowledge of the principles of the Apostles' Creed. After 1215, every Christian was required to go to confession at least once a year and most guides to confession counseled the confessor to ask the penitent if s/he knew his or her Apostles' Creed. So we have a sense that most people identified as Christian, and indeed, there are echoes of this in some dialects of Europe where "Christian" means person.

Which means that most Europeans regarded themselves as Christians. Now then, clergy were intensely worried that there might be syncretic practices creeping into Christianity. We have correspondences of bishops cracking down on "unauthorized" sites of worship. And occasionally you see things that are just plain weird. We've got, for example, the story of Étienne de Bourbon (Stephen of Bourbon), who visited a village in the Lyons area and found people venerating a Saint named Guinefort. He asked further about this saint and found out that this saint was... a dog.

There was a shrine to a dog-saint named Guinefort. The people had legends that this saint had long ago been the pet greyhound of the lord. While the lord was away from the castle, a serpent got into the baby's nursery, but the dog Guinefort attacked the snake and killed it. But during the fight, the dog had knocked the baby out of the cradle. The lord got home, saw the empty cradle and the dog with blood on his muzzle, and assumed the worst and so killed the dog. Only after he killed the dog was the baby found alive and well. But what is a saint and martyr if not someone killed for doing the right thing? So he was venerated as a saint.

Now, this story is probably a legend attached to much older legend of a dog-deity that looked after children. Indeed, the people who prayed at the shrine of saint Guinefort also carried out practices like tossing their children over a fire or invoking elves to return their child of they thought their child was a changeling. Étienne rightly noted that these practices were pagan in origin and had the shrine destroyed.

But here's where it gets important

These people thought of themselves as Christians. They saw literally no conflict between being a Christian who knew his or her Creed and praying to St. Guinefort. Or look at elves. There should be no place in Christian thought for elves/fairies, but plenty of medieval people figured that there was no conflict between believing in elves and being a Christian.

Indeed, we have a story like that of the Benandanti. In a remote region of northern Italy, you had people who believed that a baby born with a caul was chosen to fight evil witches by leaving their bodies by night. They saw no conflict between being a benandante (one of these person who had out of body fights with witches) and a Christian, and it took decades of the Inquisition torturing people into giving the "right" answers to interrogations for these people to confess to just being witches.

Which is to say that your average medieval peasant would, in general, have seen no problem with carrying out veneration of saints that bordered on out and out idolatry (as a literate churchman would define it) and being a good Christian.

A good way to think of it is that an illiterate medieval peasant would have assumed that at the top of the hierarchy of the universe was the Triune God, but of course you can pray to those other entities like saints or maybe even make offerings to fairies/elves. Churchmen didn't agree, but functionally, thinking of these syncretic practices as "lingering paganism" isn't the best idea.

Does that make sense?

Further reading:

Shinners, John. Medieval Popular Religion, 1000-1500: A Reader. 2nd ed. Toronto: Broadview Press, 2008.

This is a good selection of primary sources covering all kinds of "popular" practices of medieval Christians together with a lot of good explanations surrounding the material.

Reeves, Andrew. "Education and Religious Instruction." In The Routledge History of Medieval Christianity, c.1050-c.1530 , edited by R.N. Swanson, 103-115. London: Routledge, 2015.

A down-and-dirty guide to basic catechesis in medieval Europe.

Ginzberg, Carlo. The Night Battles: Witchcraft and Agrarian Cults in the Sixteenth and Seventeenth Centuries. Translated by John and Anne C. Tedeschi. Baltimore: Johns Hopkins University Press, 1983.

The story of the Benandanti.

Schmitt, Jean Claude. The Holy Greyhound: Guinefort, Healer of Children since the Thirteenth Century. Translated by Martin Thom. Cambridge: Cambridge University Press, 1983.

The story of St. Guinefort, the Best Boy.

Vauche, Andre. Sainthood in the Later Middle Ages. Translated by Jean Birrell. Cambridge: Cambridge University Press, 1997.

A basic rundown of the function of sainthood in the later Middle Ages.