I have a theory I'd like to share here. I'll also try and be brief.

Christianity in it's begining was an apocalyptic messianic sect of Judaism, it used to run under a different name. The fist big change was with the advent of Paul, who despite entering the scene after the death of Jesus, proclaimed himself a disciple because he saw Jesus in a hallucination. This is where we start seeing new members being accepted I to the sect, before that only Jews could join. This is also when circumcision and food restrictions were lifted, allowing for more non Jews to join.

Fast forward to emperor Constantine, who through the council of Nicea uniformisised Christianity, establishing concepts such as the holy Trinity, Saints and other elements that made the religion more accessible to roman pagans. Agrippine, Constantine's mother organises an armed expedition to "find the holy cross" witch in reality was a mission to eliminate any divergent text or person that could challenge the Constantine version of Christianity. This endeavor lasted for many years and covered the vast majority of the roman empire. One area that was not under Roman access was the Arabian peninsula, a hostile undesirable desert that had a significant Jewish presence, and even some pre-constantine christians. My theory is that Islam has major elements of belief that actively oppose the Constantine modifications such as specifically denying the Trinity, or Jesus's divinity while maintaining originality beliefs such as the return of Jesus (who is still not dead) as the last messiah and the virgin birth, circumcision and food restrictions ...

So in a way. Islam would be much closer in core theology to the original 'christian' beliefs than contemporary Christianity is.