In our opinion, the critics have failed to make the case that the jihad currently practised by ISIS and other groups is not sanctioned by Islamic texts. The critics have broken the very rules they accuse ISIS of not following when they try to make their case by ignoring abrogation and parts of Islamic texts that are inconvenient to their argument.
They have also tried to substantially misrepresent the way Islam was spread after the initial conquest of the Arabian peninsular as been by defensive wars and peaceful invitations to people to become Muslims. The historical record shows a very different picture.
Similarly with slavery, their claim that Islamic states have abolished it (under pressure from Western countries it must be said) is technically accurate, however slavery is still legal under Islamic Sharia law and it is still widely practised in several Islamic states. There is no will or movement in Islam that we are aware of to change Sharia law to abolish slavery and that such a movement is most unlikely to occur as Muhammad kept slaves himself and the Qur'an itself says that captured women may be used as sex-slaves:
[Quran (33:50) - "O Prophet! We have made lawful to thee thy wives to whom thou hast paid their dowers; and those (slaves) whom thy right hand possesses out of the prisoners of war whom Allah has assigned to thee" Also Quran (23:5-6) , Quran (4:24) , Quran (8:69) ]
The critics have also implicitly endorsed the principle of Sharia hadd punishments (Stoning, flogging, amputation) provided correct [Islamic] procedure has been followed. That these senior figures of Islam, many in the West, who purport to be moderate implicitly endorse such punishments rather than flatly rejecting is troubling.
The most troubling aspect however is that a multitude of senior Islamic figures are unable to make clear and unambiguous case against Islamic jihad and an Islamic caliphate that all, Muslim and non-Muslim alike, can clearly understand. The truth is that this letter appears to be mostly a public relations exercise designed to quiet growing Western fears regarding Islam. To that end, this letter is just another example of 'jihad by the pen' and one our governments have been quietly complicit in since 9/11 as this white paper on Reversing the Ostrich Complex makes clear.
As the article from 2013 by Tom Holland says - “It is not enough to engage with the jihadis solely on the battlefield. They must be defeated as well in mosques, and libraries, and seminar rooms. This is a battle that, in the long run, can only be won by theologians.” On the basis of this very serious effort by these critics of ISIS, we appear to be a long way from that happening, if indeed that case can be made in any unambiguous way?