On Sept. 11, 1857, a sect of fire-breathing Mormon ''purists'' led the slaughter of a wagon train of emigrants. What was the travelers' crime? According to September Dawn, a plodding and highly questionable history lesson, the interlopers weren't Christian enough. Many of them were from Arkansas (how sinful!); one of their women wore pants. The audience gets the message (religious fanaticism: bad), but nothing we see is convincing on its own not Jon Voight, in a silly beard, doing his benignly scowling, understated-overacting bit; nor the slaughter, which is all blood packs and zero horror.