Your skin color can make you marginal in some settings. Your level of income or education can do it in others. Your worship style or theological persuasion or political party can put you at or beyond the fringe in still others. Being, thinking, looking, or acting different from the majority can push you to the margins.
I'd like to speak up on behalf of a group of people in our churches who feel different pretty often, and therefore feel marginalized pretty often. Dan Taylor, in The Myth of Certainty (IVP, 2000), calls them "reflective Christians." Less sympathetic people call them doubters.
As nearly all Protestants know, in the 16th century, the Roman Catholic Church was excited about issuing indulgences—ways of reducing people's time in purgatory through religious actions, especially giving money to the church.
Despite the church's enthusiasm for it, a number of people couldn't help but question the "indulgence program." They doubted what the institution held with such certainty.