According to Google the earliest witch trials were in the 15th century. Belief in magic, however, is obviously much older.
What was the church's attitude to supposed witches prior to the 15th century? What drove the change toward intolerance and persecution?
It may be worth researching "Religion and the Decline of Magic: studies in popular beliefs in sixteenth and seventeenth century England (London: Weidenfeld and Nicolson, 1971; New York, Scribner 1971; Harmondsworth; London: Penguin, 1973; Harmondsworth: Penguin, 1978; London: Weidenfeld & Nicolson, 1997)" and Keith Thomas in general. Magic was a part of the universe and both the Church (the institution) (in the form of ringing bells to drive off storms) and the church (the congregation in innumerable forms ) used magic extensively.
Reviewing the other answers, I think they rely overly much on Exodus. This is weak historiography. The quote from Exodus is presented without context, and without any analysis to determine whether the quote aligns with the rest of the source material (a source that includes other quotes such as "thou shalt not kill"). I don't intend to engage in hermeneutics; my point is merely that it is not good history to base an entire analysis on a single quote.
As Thomas mentions, the institutional church was permeated by magical practice - the clearest of which is the ringing of church bells to avert storms.
The ringing of church bells is not documented in the Bible - it has no scriptural support, but it was ubiquitous and supported by the church. Most medicine was also not supported by scripture or science but was part of the Church.
William of Auvergne, a 13th-century French priest and bishop, certainly condemned most magic as superstition. However, he admitted that some works of “natural magic” should be viewed as a branch of science: as long as practitioners didn’t use this “natural magic” for evil, they weren’t doing anything criminal. Sealskin could quite happily be used as a charm to repel lightning; vulture body parts could be used as a protective amulet; and gardeners could get virgins to plant their olive trees without any anxiety – this was, after all, a scientific way of promoting their growth. HistoryExtra.com
or
Many unofficial rituals and beliefs existed alongside ones sanctioned by the Church. Educated clergy condemned some as magic, but it wasn’t always easy to do this because many magical and superstitious practices employed religious language, rituals or objects. Charms recited over the sick to cure illnesses often invoked God and the saints; spells for love and other purposes might use consecrated substances such as the Eucharist. The people reaching for them could even justify their actions by citing biblical precedent. Magic and Religion in Medieval England
or
The Lacnunga prescribed a set of Christian prayers to be said over the ingredients used to make the medicine, and such ingredients were to be mixed by straws with the names “Matthew, Mark, Luke, and John” inscribed on them. In order for the cure to work, several charms had to be sung in Latin over the medicine. Wikipedia:Medieval Magic There are multiple other examples in this article.
Cursory research would reveal dozens more similar practices. The church congregation practiced superstition and folk magic commonly and the church made no effort to stop them. (Again, cursory research would reveal dozens of examples)
What is the distinction between practices that are not supported by science or scripture but are expected to be effective practiced by the Church and practices that are not supported by science or scripture practices by the congregation, and practices that are not supported by science or scripture that are suppressed by the church?
Clearly the distinction is not about the practice - it is about whether the practitioner supports or weakens the Church. The Church supports and participates in the first two categories, but opposes the third. (this is directly responsive to OP's question - took me a moment to get here, but I had to cover some fundamentals). The church always suppressed those who engaged in practices detrimental to the community, whether those practices were simply criminal or magically criminal.
But at some point in the 15th century something shifted and the church demanded a monopoly on magical practice. That is the real question, and unfortunately I don't have an answer, although I think there is a very strong clue in the history of the Benandanti. Much of what we know today was invented by the Inquisition, but a quick review of the primary source literature reveals two things. First, the image of heroic female warriors riding bunny rabbits into battle wielding bulbs of fennel - an image that simply should not vanish from our collective memory. And second that there was clearly a folk ritual known and accepted among these cultures that involved magical practice to benefit the community that was not perceived to be evil until the Inquisition offered to stop torturing people in exchange for testimony that would benefit the Inquisition.
I think that is the first answer to OP's question; when persecution became an institution, the first law of institutional behavior took over - "Whatever is good for the institution is good; whatever does not strengthen the institution is suspicious" - note that I admit this is my opinion.
K. Thomas argues that in England the persecution arises from other purely political causes which it was convenient to mask under persecution of magic - kind of like dogwhistle politics, where one term is understood by members of the faction to represent another concept that it would be embarrassing to oppose. He makes a convining point that the rise in the persecution of witches was the result of Enclosure.
I'm afraid that it has been 20+ years since I've read Thomas or researched medieval attitudes towards witchcraft, and I haven't kept current, but I hope that will provide you an armature for research.
"Witchcraft" and magic were viewed by the Church as religious crimes, (making a pact with the Devil to obtain these powers), and therefore treated harshly.
Note, however, that this trend had barely begun by the 14th and 15 centuries (a two century period when this "transition" took place). Prior to the 14th century, there was no such linking of extraordinary powers with the Devil.
The Church was primarily concerned about prosecuting "heresy," and did not make a link between heresy and witchcraft until about the 14th and 15th centuries. Instead, magic was earlier considered "superstition," a lesser issue.
It is worth considering the etymology of the word magic, from 3rd century Latin magica. Now look at that of magus from the Classical Latin - meaning "wise man", and we see there is at least a linguistic connection from the magi to magic. There is an inevitable nexus between the origins of religion and those of the occult.
As regards witches, in England the centuries of the female witch were the 16th and 17th. The historiographical reasons for this are embedded deep in social history. One of them has to do with it being an age when access to legal process was becoming more available to the public..
The sixteenth century was also a time when traditional village and family support for lone persons, particularly females was breaking down, as society became more mobile. Workhouses and the poor law were introduced. Lone people remaining outside of provision were isolated as traditional family support broke down - the theory goes. Such lone individuals could become figures of suspicion. There had been far fewer cases of witches before the sixteenth century.
One of the leading historians in this field is Dr Clive Holmes, Emeritus Professor of Social History, Lady Margaret Hall, Oxford. Holmes explains why trials for witchcraft ceased in Britain after the seventeenth century - the last conviction was in 1712 - though there were trials well into the 19th century. See Holmes' Why did the prosecution of witches cease in England? Kindle edition.
As I said in the comments above, this isn't one of my specialist areas, but a few points may help provide an answer.
Firstly, given the exhortation in Exodus 22:18 "Thou shalt not suffer a witch to live", we are probably on safe ground if we say that the attitude of the church towards practitioners of "magic" wasn't particularly positive - even before the 15th century.
Now, I know that there is a debate about whether the Hebrew term mekhashepha, translated in the King James Bible as "witches", should be better translated as "poisoners". However, since the mekhashepha (whoever they might be) are also included with "necromancers", "those who cast spells", "those who summon spirits" etc., as "an abomination to Yahweh" in Deuteronomy 18:9-10, I think it is safe to assume that the church would have taken a strong stand against them - whatever debates among modern scholars may conclude about the precise meaning of the original Hebrew word.
In fact, we know that the early church took such a stand against them. They actually passed decrees which condemned magic:
"... not only preached against magic, but also passed ecclesiastical legislation against it. The decrees (or "canons") of regional assemblies (or "synods") eventually became the basis for the church's "canon law", which even in its early forms condemned magic.
So it seems clear that the church's attitude to magic, as expressed in its early decrees, and later in canon law, was a highly negative one.
What drove the change to intolerance and persecution?
As J Asia has already pointed out, there really was no change in the 15th century.
Witch hunts were by no means unknown in the Middle Ages. However, the growth of the Inquisition from the 13th century (initially in response to Catharism), and the development of the Reformation from the 14th century onward meant that religious crimes and heresy were being dealt with increasingly harshly.
Of course, one other significant factor in the later persecutions was King James I/VI himself. We know that he was obsessed with the threat posed by witches. His book Daemonologie would become a key text in later witch hunts.
Source:
Kieckhefer, Richard: Magic in the Middle Ages, Cambridge University Press, 2000
"What was the church's attitude to supposed witches prior to the 15th century?"
Presumably it's intolerance of magic, as outlined in Exodus 22:18. The problem is that it's a form of violation of the First Commandment as given, "You shall have no other gods before me." Likewise it's a violation of the Greatest Commandment given in Matthew 22:37. That is, trust in God and him alone. Trusting in magic indicates not trusting in God. Israel at that time was a theocracy established by God, but nations since that time are not. So the treatment of someone thought to be a witch was a matter of law before Jesus in the nation of Israel, but such treatment is no longer permitted. Nevertheless, that hasn't necessarily been understood by all in the church.
"What drove the change toward intolerance and persecution?"
Intolerance seems to be a theme of our human nature. That is, we are intolerant of those that are different. For example, witches, Confederates, blacks, Christians, Muslims, etc. Persecution builds on that intolerance through the belief that it's okay to persecute those we cannot tolerate. That is, that if we find someone abhorrent, such as a racist, we then perceive that we somehow have the right to persecute that person. It may be difficult to look at someone else's intolerance and wrap our heads around what makes them tick. It's probably easier to contemplate our own intolerance of someone we find abhorrent (maybe against racists is a good example) and that will help illustrate what drives us, and shed light on similar patterns in others.
A Timeline on 'Magic' and Witch-Craft in the Middle Ages, and the 'Church'
"What was the church's attitude to supposed witches prior to the 15th century?"
Answer: Exodus (22:18) says: “Thou shalt not suffer a witch to live.”
What drove the change toward intolerance and prosecution? Answer: There was no change.
What could have caused the increase in witchtrials during the Middle Ages?
(not asked, but I'm presuming this is of interest).
A few reasons, well-documented in Witchcraft in Europe, 400-1700: A Documentary History -- Alan Charles Kors, Edward Peters, University of Pennsylvania Press, 2001. I will go with Little Ice Age, as one of the major factor. Scientific analysis and correlation (not causation) with witchtrials is published here: Witchcraft, Weather and Economic Growth in Renaissance Europe, Journal of Economic Perspectives, Winter 2004. From the abstract:
In the period between 1300 and 1800 as many as one million people were executed in Europe for the crime of witchcraft. Although a variety of theories about the witch-hunts have been put forward over time, little has been said about the possibility that the witch-hunts were motivated by a desire for vengeance in a time of misfortune. This paper connects the witch-hunts in Europe with deteriorating weather and slow economic growth during this period. The most intense period of witch-hunting coincided with a period of below-average worldwide temperatures known as the little ice age.
NOTE: I don't deny that the 'Church' did not always follow Exodus (22:18) but it does not mean a change when they do, because the witch-trials were (in my opinion) never truly about ideology - rather, it was clearly driven by material-wealth (i.e. economic returns).
