Christian influence on Islam pre-630 CE?

Several Byzantine chroniclers (and I will have to dig through my library for their names) considered Islam to be a Christian Heresy in the early Eighth Century. It also ahs a great deal in common with early Rabbinic Judaism.

What exactly was it that was similar to Rabbinic Judaism? Also, how could Islam be a schism of Christianity? It doesn't even consider a combination of divinity and Jesus. I don't know, I'm just asking.

the linking thread of judaism>christianity>islam is Abraham, the personage of jesus is considered an apostle in Islam (perhaps not the right label) actually i think the term used in the koran for jesus is prophet

From what I've read (and Medieval Islam is only a minor field, my major field is Western Medieval religious and Intelellectual) the similarities in legalities quoted in the Koran to Jewish orthopraxis in late antiquity lend credence to the theory that Islam may have been heavily influenced by Judaism. Bear in mind the prevalence of Jewish merchants and Jewish settlements in coastal Arabia at the time.
One would think that the reason Byzantines would assume Islam was a Christian Heresy would have been the lack of cognitive models for anything else.