The doctrinal emphasis on the idea of penal substitution that so typifies Western Christianity may date from about 1100 – 1200 CE, that is following the Great Schism of 1054 CE. That schism allowed Western Christianity to develop along different theological lines to the Eastern traditions. The idea of penal substitution does occur before, but is is not doctrinally central. In the Eastern traditions it has never gained much adherence. The schism provided extra impetus for an overhaul of the Western traditions, an impetus that may have started with the Cluniac reforms. Church beliefs and practices were de-Paganised (the period 500 – 1000 CE can be thought as one of Paganistic-Christianity in Western Europe), Marianism was fostered and a much more penitential, lapsarian doctrine espoused.
The Reformation developed the emphasis on penal substitution further, especially in the light of 'original sin' and its lapsarian consequences. This is particularly clear in the works of John Calvin. Modern fundamentalist beliefs seem to be more in tune with Calvin's developments than with the beliefs of the early church or of the traditions of Eastern Christianity.
Traditionally Unitarian thinking (both post-Reformation and Early Christian) denied the divinity of Jesus, seeing him as fully human. The resurrection often being seen as metaphorical and spiritual, not physical. The story of Jesus being seen as a metaphor for all human suffering and its spiritual transcendence, something only fully possible if Jesus was purely human. For Unitarians it was Jesus's humanity, not his substitution, that gave the Passion its special relevance.
Is Unitarianism a somewhat Islamic view of Jesus? Why not? It would not alter one word of the Gospels, simply the way in which you see them. Perhaps it is no accident that modern Unitarianism arose first in Transylvania, an area that had been part of the Ottoman Empire and subject to considerable Islamic influence and thought.