I have no problem with religious people — or political people for that matter — who claim a monopoly on truth as an over-enthusiastic commitment to their faith.
Or as a hyperbolic assertion of dedication to its content.
But when monopoly-on-truth is taken literally it always contains within itself a lethal germ of murder at best or even genocide at worst. And it usually moves from the ideological, through the rhetorical, to the physical level.
First, such people may say, with utter sincerity, that error has no right to exist and that wherever it appears, it must be destroyed one way or another.
Second, when such people achieve power, they often act against those who promote error in order to remove, silence, marginalize, or even execute them.
Finally, those who claim a monopoly on religious truth know that, if all error-mongers are dead, their own claim would be factually correct.