Deism is the belief in an impersonal God who created the universe and the natural laws that governed it, but who them stepped back and let the universe run according to those nature laws, not through arbitrary divine intervention. It developed in Christian Europe during the Enlightenment and continues to this day.

European Medieval Religion:

Christians in the Middle Ages (about 6th through 13th centuries CE) saw the world as ultimately controlled by supernatural forces, whether that be God, demons, or magic.

The divisions between science and magic that we make today were non-existent in the Middle Ages. No distinction was made between a root a healer told you to eat and a root a healer told you to hang over your door.

Renaissance Science and Supernatural Beliefs:

Renaissance literally means "rebirth." Renaissance intellectuals were rediscovering Greek and Roman writings and were re-embracing many aspects of Greco-Roman culture, including the study of the natural world.

However, this is also the same time as the Reformation and great religious tension between Protestants and Catholics. Various social and political changes were happening as well, and the stress of all this helped lead to the European Witch-Craze. Between the late 15th century and the mid 17th century tens of thousands of people were executed for witchcraft, and many more were accused. Ironically, just as Europe was starting proto-scientific investigations it was also the most paranoid about supernatural events. Contrary to popular belief, most accused witches died in the Renaissance, not the Middle Ages.

Ages of Reason and Enlightenment:

People generally talk about the Age of Reason starting about 1600 and the Age of Enlightenment starting about 1700, although others use the terms interchangeably.

Two big things happened in the 1600s. First was the Thirty Years War, which killed several million people and which started over religious divisions. Europe came to accept that bullying the other side into conformity simply would not work. The second was the end of the Witch Craze, in part because of the growing acceptance that witchcraft accusations were generally both unprovable and impossible.

Meanwhile, we see continuing growth of proto-scientific study. Some of it has now been categorized as pseudo-science (such as alchemy), but all of it was based on attempts to investigate and understand the world around us without resorting to supernatural explanations.

Rise of Rationality:

For these intellectuals, reason became the supreme intellectual authority. If you were to believe in something, you should be able to rationally defend it, whether scientifically or philosophically. If someone is physically sick, there should be a physical cause of the ailment. (We don't correctly figure out what those causes are until the 19th century with germ theory, but the idea that there should be a physical cause had been around for a long time by that point.) Blaming it on demons or the will of God was irrational because there was no evidence to support such a claim. Instead, such claims were based on long accepted truths put forth by people in positions in authority.

Suspicion of Authority:

But if you're going to depend ultimately on your own rationality rather than an authority, what does one do with the Bible? There is no way to prove many of things in the Bible. It is a statement of things as fact which Christians had been accepting for centuries.

On top of which, many people were growing increasingly hostile to the clergy. They saw Christianity as becoming increasingly confused and corrupted by those wishing to remain in power and profit from such positions. It was the priests who told people to trust the Bible, as well as their own understandings of the Bible. That was a necessity of the early Middle Ages when almost the entire population of Europe was illiterate, even nobles and kings. But now a sizable minority of the population was educated, and there was far less of a need for interpreters.

Past 1800:

The 18th century was the primary period of deism. Past 1800, the number of people identifying themselves as deists declined. Some joined the Unitarian movement. Others became atheists or agnostics, continuing to value rationality but questioning whether a divine creator was in any way necessary for rationality to exist. Still, there are people who still identify as deists today.