The Atlantic has published a really valuable article for understanding ISIS.

One thing that makes Protestantism superior to Catholicism is the burning of heretics.  Now of course the Protestants burned heretics along with the Catholics, but a Protestant can denounce the action, while a Catholic can’t.  Heretics were burned by order of popes and councils, for example the Council of Constance which burned John Huss to death.  And in Roman Catholic theology, councils of the church are infallible.  I can say that the Genevan church was wrong to burn Servetus.  But the Roman Catholic can say no such thing about the Council of Constance.

Muslims have a similar problem.  No matter how modern and moderate a Muslim becomes, he cannot escape the example of Allah’s true prophet, who held slaves, who married a nine year old, who practiced all sorts of barbarities.  The Islamic State says that anyone who rejects crucifixions and slavery rejects Islam, because it is to reject the practice of Muhammad and the teachings of the Koran, and they have a point.  As Christians, however, we can never go wrong by returning to the example of our founder, Jesus, whose values and teachings have shaped the modern world, values by which the members of that modern world judge people like ISIS even if they do not acknowledge the source of those values.

The Christian church at its very best attempts to emulates the true principles and practices of its founder.  Muslims at their best ignore the values of Islam’s founder and adopt the values of the founder of Christianity instead.