As her name suggests, Mercy is a kind and loving soul, a woman of infinite patience ready to help others as best she can. Mercy's disability has confined her to the home, which has inculcated a mindset of calm passivity. To some extent, it also influences her faith. Though a Puritan, Mercy is not an intolerant fanatic like so many of her co-religionists. One certainly can't imagine her participating in any witch-hunts, for example.
In chapter 4, when Kit's Aunt Rachel goes to fetch the Widow Brown some leftover corn bread, Judith complains bitterly. She doesn't see why her mother should have to do everything for her. Mercy responds to Judith's lack of charity by gently reminding her what the scriptures say about caring for the poor and the widows. For Mercy, Christianity is about caring for and loving other people, not judging and condemning them.