IT is probably not to his advantage that Mitt Romney’s clean-scrubbed, youthful presence so readily reminds voters of those earnest Mormon missionaries knocking on their doors.

If it were almost any other church, a missionary past would most likely be an asset for a presidential candidate. To have spent two years in mission work after high school is a sign of early and admirable idealism, commitment and strength of character.

But to many American Christians, those friendly Mormon missionaries embody exactly what they fear and resent about Mormonism. And Mr. Romney, after nearly a year of graciously sidestepping invitations to a theological duel, may have engaged it despite himself last week by giving a speech on faith the likes of which hasn’t been heard since John F. Kennedy took on voters’ fears of Catholicism in 1960. While Mr. Romney uttered the word “Mormon” only once, he jump started the discussion about what makes Mormonism problematic for some Americans.

From the start of the Church of Jesus Christ of Latter-day Saints in upstate New York 180 years ago, missionaries have been spreading the message that Christianity lost its way and Mormonism restores the Christian church to its rightful path. The first prophet of Mormonism, Joseph Smith, essentially threw down the gauntlet to the other churches. He was killed at the hands of a mob.