Islam already made inroads when Vatican II taught that they worship the same God as we do.
Vatican II?
The following is not from Vatican II:
“3. In the West, there is the letter of Pope Gregory VII (1020-1085), written in 1076 to al-Nasir, Prince of Bougie (in Algeria), who had sent the priest Servandus to him to be ordained bishop. Over and above of the political context, the letter remains relevant: ‘You and I owe this charity to one another even more than we owe it to other people, as we recognise and profess - in different ways, it is true - the One God, whom we praise and venerate each day as the creator of all and master of the world, according to the words of the Apostle: For he is our peace,; in his flesh he has made both groups into one’ (Eph 2,14). http://www.cec-kek.org/pdf/MeetingMuslims.pdf
The quote from Gregory VII is undoubtedly genuine even if that link is not dead because it is cited in Vatican II documents as coming from PL. You can always look that up online if you so choose: St. Gregory VII, Letter III, 21 to Anazir [Al-Nasir], King of Mauretania PL, 148.451A. Also, there is an updated url: http://cid.ceceurope.org/fileadmin/filer/cid/Doc_Interreligious_Dialogue/MeetingMuslims.pdf
Another version of the quote floating around online is:
“For there is nothing which Almighty God, who wishes that all men should be saved and that no man should perish, more approves in our conduct than that a man should first love God and then his fellow men … Most certainly you and we ought to love each other in this way more than other races of men, because we believe and confess one God, albeit in different ways, whom each day we praise and reverence as the creator of all ages and the governor of this world.”
So you think Vatican II took place in the 11th century, huh?
By the way, Pope Gregory VII considered a crusade - it would have been the first - but was unable to pull everything together because he was so occupied with difficulties in Church reform and problem with the HRE.