" When someone is comfortable with what they believe (it seems to me) that they feel little need for "vehemence"."

Well, John Hardly, I see the point...but I also at least think I see why some DO feel strongly...and often vehemently..about the issue.
MMario noted that "faith is illogical", a position treated in great detail by philosopher Sören Kierkegaard in "Fear and Trembling" and other works. Since history is full of abuses, wars, hate and intolerance done in the name of religion, there are many who get very irate that what they consider an 'illogical' position to begin with can lead to so much suffering. "Why", they say, "if God exists, and is so great, does so much evil happen?"
....Well, we know that there are various theological answers to that oft asked question...and we also know that a lot of good is done through religion....but these days, when there is a great deal of ill will being espoused and violence being done, fueled by religious fervor, I can see why some would be tempted to "vehemence" in their opposition.

I had a friend tell me once, many years ago, that "the conflict in Ireland is not 'about' religion"....but I replied to him.."yeah, but it sure is the identifying marker when they want to shoot or bomb each other"....

it is simply that some think that(if they think at all), if logic and reasonable debate do not make men aware of the folly of their positions, then 'perhaps' vehemence will get their attention