There seems to be an inverse relationship between how vociferous believers are in claiming that their religion is peaceful and how peaceful their religion actually is. Perhaps a truly peaceful religion is obviously peaceful and doesn't raise a lot of red flags, so adherents don't need to go out of their way to say how peaceful they are. Violent religions, though, have a PR problem with outsiders so adherents need to go out of their way to explain how peaceful their beliefs actually are.
Christians can be especially critical of how Muslims keep insisting that Islam is a "religion of peace" despite the extensive world-wide violence being committed by Muslims in the name of Islam. Such Christians seem to want to insist that theirs is the real "religion of peace" because Jesus is the "prince of peace." Historically, though, Christians don't really have much advantage over others — Christians have had little trouble engaging in religious war against others.
The quote above, "Kill them all; for God knows His own" is usually rephrased as "Kill them all, God will sort them out." It was attributed by Caesar of Heisterbach, a papal representative, to Arnaud-Amaury, Abbot of Citeaux and military leader of the Cathar Crusade, during the abbot's sack of Beziers in southern France. Around 10,000 residents were massacred because the city had been officially allied with the Cathars, a Christian heresy. This means that this infamous statement was made by a Christian leader in the process of slaughtering Christians whose beliefs differed from the officially-approved beliefs.