The major issue is that while they indeed share a common heritage, these can lead to vastly different values. Judaism, for instance, puts far more value on works in the material world because there either is no afterlife, or if there is it's not a meaningful one, whereas Christianity sees the afterlife as the life, the true life, beyond the mere distractions that the material world tries to impose upon us
Islam sees that tradition too come to a very different end ideologically. Certainly not "opposed" as none of the major religions have truly opposite ends (since they all promote some sort of universal goodness to the point where if we all obeyed the basic tenets of religion without getting into any of the technical crap, we would generally be at peace), but the ideological paths are skewed, and were skewed from the beginning of Islam (and of Christianity)
Muhammed was very different from Jesus, but one must understand that the world he worked in was a sight more savage, certainly more tribal, more of a dog-eat-dog world which is why you see Islam with some of its stricter tenets than Christianity.
Christianity got skewed too, with the function of the afterlife, and the spirit being somehow more pure than flesh, coming in from Greek philosophy, made possible by the fact that Judea was run by the Roman Empire
The Caliphate also bound Islam more closely to the state, which was also done with Christianty, but in a less permanent way (because they understood overall that the idea of the state predated Christianity, unlike in Arabia where central government was often nonexistant before the Caliphate rolled in, and why certain Muslim-majority countries have had far less trouble adapting to a secular model of rule than otheres)
Though i agree with the sentiment of your post at heart
Monster Hunter: pissing me off since 2010.
