I have only read this statement in a chick-tract some years ago; it is however speculative and no sources are provided.
Chick mainly points out that the Vatican made an alligiance with Muhammad before Islam to bring the tribes on under the Catholic faith; this cannot be confirmed however. But then suddenly Muhammand founded Islam.
There is no doubt that Muhammad had much contact with Christians in Arabia, both in Mecca and Medina, there were in fact Christian intellectual centres of theology and science all over, particularly by the Syriac and Nestorian Christians as well as the influence Abyssine Christians.
Furthermore, Christian sects were flourishing in the dessert, particuarly Gnostic and groups that leaned toward docetism; this is very obvious in the Qur'an; Jesus only appears to die on the cross, this was a Gnostic idea since the divine Christ who possessed the human and physical man Jesus simply left his body and took off to heaven; there are other ideas evident too.
Also the entire embryology idea in the Qur'an derives from Galen, the Syriac Christians who flourished in Arabia were particularly found of Galen and embryology, they possessed as far as I have heard whole schools of embryology; no wonder this is found in therein.
We also have the separation of heaven and earth mentioned in the Qur'an which was a famous Greek idea, a standard module for the creation of the universe (see Lucretius; the Nature of the universe); there is also reference to the 'seven heavens and seven earths', a typical Jewish idea taken from the Talmud, and idea which the Jews had combined with Greek philosophy (in fact do a study of Qur'anic science and then read through the ancient philosophers, this will prove to have a significant effect; and it certainly indicates borrowing and corruption).
Beside we have all the stories from Jewish tales and even Christian tales, which are nothing but tales in the first place.
And all this is only a drop in the sea.
So yeah there was a strong Christian and even strong Jewish influence upon Muhammad, but there is no evidence that the Catholics or any other Christian faction created the religion of Islam.
It seems more that Muhammad and a number of Syriac backsliden Christians sat down and fabricated the Qur'an; this is indicated in the Bukhari and from various Qur'anic passages, as I have already pointed out here. It seems therefore to be a mixture of Christian gnosticism, Nestorianism and Talmudic Judaism, mixed with pre-Islamic Arabic religion.
The later crusades were a response to Islamic invasion of Europe and Christian territory, such as Syria, Palestine, North Africa, Spain, France and the Balkans; while the crusades were nasty and hiddeous things happen, that is just how war is, and as Christians we should rejoice about it, it is totally illogical to expect the habitants of Europe to sit and just watch their territory be conquered. However, this repelling strike did not create the religion of Islam either.