Christians who believe that the nature of Jesus is ONE both human and divine but not THREEDivine, Human, Spirit.
This belief was most accepted among the Arab Christians of Syria especially, and of the Middle East in particular. These Christians were mercilessly persecuted by their 'brother' Greeks who believed in the Trinity.
Although the Christian Arabs under their kings, al Harith, his son al Mundhir and his son al Nu'man, tried their best to come to a modus vivendi with them, but were systematically betrayed by the Greek clergy to such an extent that they sided on several occasions with the Persians.
In the end,when the Muhammadan Arabs came to conquer Syria, Palestinia and Egypt, their Christian sects welcomed them as liberators and deliverers compared to the oppression they were suffering at the hands of the Byzantines.
This is the most important factor that allowed the Muhammadans to sweep with such little resistance and incredible speed in their subjugation of the Middle East and Egypt.