Before gunpower, "barbarian" empires (Jurchen, Mongols, Turks, Arabs, Berbers,etc.) were established according to the following patterns:
1. Charismatic leader unites a barbarian confederacy. Combined maybe with a population explosion, or a population depression in the adjacent civilized areas, and the barbarians being generally more proficient militarily anyway, they curb stomp the neighboring civilized states and create an empire.
2. Civilized states recruit soldiers from the neighboring barbarian areas, and eventually these recruits or mercenaries take power in the civilized areas.
3. A state that is on the civilized/ barbarian borderlands expands into the civilized core during a period of turmoil in the civilized area.
Of these, method # 2 was by far the most common. The 7th century Arab conquests followed more the pattern of #1. The classical era short lived Arab empire, Palmyra, followed more the pattern of # 3.
Given the situation in the seventh century, I think the Arabs will become more prominent, but it will follow the usual patterns of # 2 and # 3. The situation with the East Romans or Byzantines will be more of # 2 and follow the Palmyrene model. An Arab state will be established in Syria at a time when Constantinople can't do much about it, and expand into Egypt. With the Sassanian empire, I think its more likely that # 3 will happen, that the ruling dynasty will be overthrown by its Arab mercenaries, and replaced with an Arab dynasty in what is essentially an Arab -Persian empire, with maybe the Sassanians continuing as figureheads. And they may then conquer some Byzantine territories.
This wouldn't preclude a new Abrahamic religion arising later, either from Manichaeism, Zoroastrianism, or Monophysite Christianity, and this religion would look a lot like Islam. However, without Islam, Arab or Arab-Persian expansion into the Sind, Central Asia, North Africa, Spain, and Sicily is obviously much less likely, and there is no Islamic missionary activity, which IOTL was considerable and converted lots of areas that never saw an Arab army.
1. Charismatic leader unites a barbarian confederacy. Combined maybe with a population explosion, or a population depression in the adjacent civilized areas, and the barbarians being generally more proficient militarily anyway, they curb stomp the neighboring civilized states and create an empire.
2. Civilized states recruit soldiers from the neighboring barbarian areas, and eventually these recruits or mercenaries take power in the civilized areas.
3. A state that is on the civilized/ barbarian borderlands expands into the civilized core during a period of turmoil in the civilized area.
Of these, method # 2 was by far the most common. The 7th century Arab conquests followed more the pattern of #1. The classical era short lived Arab empire, Palmyra, followed more the pattern of # 3.
Given the situation in the seventh century, I think the Arabs will become more prominent, but it will follow the usual patterns of # 2 and # 3. The situation with the East Romans or Byzantines will be more of # 2 and follow the Palmyrene model. An Arab state will be established in Syria at a time when Constantinople can't do much about it, and expand into Egypt. With the Sassanian empire, I think its more likely that # 3 will happen, that the ruling dynasty will be overthrown by its Arab mercenaries, and replaced with an Arab dynasty in what is essentially an Arab -Persian empire, with maybe the Sassanians continuing as figureheads. And they may then conquer some Byzantine territories.
This wouldn't preclude a new Abrahamic religion arising later, either from Manichaeism, Zoroastrianism, or Monophysite Christianity, and this religion would look a lot like Islam. However, without Islam, Arab or Arab-Persian expansion into the Sind, Central Asia, North Africa, Spain, and Sicily is obviously much less likely, and there is no Islamic missionary activity, which IOTL was considerable and converted lots of areas that never saw an Arab army.