This is a period movie based on the historical folklore of early colonial America.
Nearly all of the lines of dialogue is taken from colonial folklore books and from the diaries of real-life colonial Americans. It's easy to see how the superstitions of the time gave way to the things like the Salem witch trials.
The movie a slow burn that is a little bit creepy, but it isn't scary in the traditional sense of the word. It's fascinating, and its scares are more psychological.
People can be evil, and people are generally naive and gullible. The combination of those two traits has destroyed a lot of lives.
One reviewer said that watching The Witch is like witnessing something we shouldn't be seeing.
That's a good description.