Though set in the 17th-century, the language is as straightforward and direct as the Puritans of New England themselves.
At the same time, the tone is sympathetic. Many characters find themselves misunderstood, in one way or another, but the narrator's tone always tends to gently highlight something positive beneath the surface. In a book full of judgmental characters, the narrator prods us to look deeper, and not fall into that trap. Here's an example of the narrator describing Prudence:
A more unpromising child she had never seen, Kit thought, yet she couldn’t get Prudence out of her mind. There was some spark in that small frame that refused to be quenched. (2.9)
And here's an example of the narrator on Uncle Matthew:
Going through the shed door one marooning, with her arms full of linens to spread on the grass, Kit halted, wary as always, at the sight of her uncle. He was standing not far from the house, looking out toward the river, his face half turned from her. He did not notice her. He simply stood, idle for one rare moment, staring at the golden fields. The flaming color was dimmed now. Great masses of curled brown leaves lay tangled in the dried grass, and the branches that thrust against the graying sky were almost bare. As Kit watched, her uncle bent slowly and scooped up a handful of brown dirt from the garden patch at his feet, and stood holding it was a curious reverence, as though it were some priceless substance. As it crumbled through his fingers, his hand convulsed in a sudden passionate gesture. (14.2)