Not only the conflating of destiny and omniscience, which in Calvin's case destroys men's souls by making the only Savior into a capricious monster, but other expressions of Islamic barbarism had to have warped the Christian culture it oppressed for centuries. One institution completely alien to the Scriptures -- but in line with Allah's malevolence -- was the Spanish Inquisition.
Though its evils have been exaggerated by those with an axe to grind against Christianity and Roman Catholicism, and though it was government that did the dirty work, the idea that people should be punished temporally for belief is foreign to the New Testament texts, but inherent to Islam's.
Bad theology always causes problems.
On Muhammad's influence on Calvin, from the great Diana West:
One of the most fascinating essays I've ever read about Islam in all these many years since 9/11 (when I first started reading essays about islam) is in the essential book, The Legacy of Jihad, by Andrew G. Bostom.
It is by the French theologian Jacques Ellul and it is called "The Influence of Islam." The essay discusses the influence of Islam on Christianity -- namely, what was "imported into Europe" from Islam that took root in Christianity.
Among other things, Elllul links the elevation of canonical law, the entrenchment of the divine right of kings, and the emergence of "holy war" as evidence of what he notes was a one-way cultural exchange propelled by contact, competition and war with Islam. Islamic influence was an import into Christendom, but there was no reciprocal export of Christian influence into Islam.
Another aspect of this influence that I remember being quite struck by when I first read the essay some years ago had to do with the emergence of Christian notions of "providence," which, as Ellul writes "is never a biblical word" or concept, but was helped into Christian doctrine by the Islamic concept of submission, particularly the Muslim formulation mektoub, "It was written." He writes:
From now on destiny and divine omniscience are conjoined. Believers can live in perfect peace because they know that everything was written in advance. The very formula "It was written" could only come from a religion of the book. Yet the Hebrew Bible and the Gospels never use such a formula. Thanks to it, the idea of predestination that was already haunting philosophical and Christian thinking received confirmation, forcibly established itself, and came to include double predestination (in Calvin), which, whether we want it or not, transforms the biblical God into destiny, Anamke, etc. And this derives from Muslim thinking.