A few small points. 1) Muslim countries are taking the lead in attacking ISIS, from UAE, to Egypt, to Jordan, to Indonesia, to Malaysia, to (although not very good at it) Pakistan and beyond.
2) I live in the middle east (as an expat, doing some consulting work with governments across the region), and I can assure you, this 'sizeable minority' claim is not present in the vast majority of the Muslim countries I work in. Walk down UAE and support ISIS? You're liable to be arrested for promoting extremism.
3) There are countless contemporary cases of Christian-inspired attrocities, from genocide to blatant terrorism, and yet we (fairly) do not attribute these lunatics as representing all Christians.
Such as:
Oklahoma City Bombing, (inspired by the Waco siege and the davidians)
Utoya attacks in Norway and the perpetrator's 1500 page anti-immigration and anti-islam manifesto; remember, he killed 77 people including many children
the Lord's Resistance in Uganda,
the hundreds of abortion clinic bombings in the US over the last few generations,
the centuries-long disputes, murders, assassinations and terrorist attacks by both catholic and orange(protestant) militia in Ireland continuing right up until the 90's (and still happening in less pronounced degrees),
the Manipur ethnic cleansing conducted by the Nationalist Socialist Council of Nagaland who's slogan was, "Nagaland for Christ",
the NLFT atrocities in North-Eastern India in the 90s, and etc, etc etc.
https://en.wikipedia.org/wiki/Christian_terrorism#Contemporary
4) People seem to forget that there are Christian sects that condone very similar extreme rules over dress, role of women (banning them from working), to polygamy, and other perspectives that are not 'aligned with liberal democracy', like Amish and Mennonites, yet we find a way in our 'liberal democracy' to accommodate them.