The definition of Arabs and their ethnicity is, as I understand it, largely language and culture based, rather than genetic. In other words, Arabs are the various peoples of the middle east who, either themselves came from the Arabian Peninsula or were Arabized during the various conquests or, and most commonly, a mixture of the invaders with the locals. Arabized basically means the overlay or replacment of the exisiting languages and culture with the Arabic language and the Muslim religion. Christian Arabs are, for the most parts, remnants of the indigenous pre-Islamic invasion religion in people who adopted the Arab language but not the Muslim religion (because Muslims cannot convert to Christianity, generally speaking, marraiage was often a one way street so mixed couples become Muslim – as a consequence, the Christian populations tend to be closer to the original, indegenous, pre-invasion population, e.g., the Copts in Egypt) . Obviously, even in Muslim Arabs, not all elements of the pre-existing culture was wiped out and ethnic distinctions still remain as do pockets of indegenous language. A Palestinian Arab friend once sent me an article about a very old sub-group of Palestinian Muslim Arabs who still kept remanants of some of their pre-Muslim Jewish rituals – specifically matzah-like crackers in springtime (if people are interested, i can try to dig it up). In fact, many Palestinian Arabs, Muslim and, obviously, Christian, are descended from the indigenous Jewish population that remained after the Roman dispersal (by descended, I dont mean exclusively so, just that they were part of the mixture, along with Romans who stayed and migrants from other parts of the region and indegenous non-Jewish cultures). Throw on top of this, multiple waves of colonialism, first Arab, than Ottoman, than Western and you get lots of mixing. All of this is painted in very broad strokes, but is a reasonable if gross description. Certainly, different locations and sub-cultures have different, more specific version of the story.
Since people here seem to like talking about Iran, one thing to note is the significance of the fact that while the original Persian religion was wiped out and replaced for the most part with Islam, they were still able to retain their language. Presumably, that has great cultural significance about which we can all speculate.
Anyhow, this is an outsiders perspective. I would love to hear what the various Arabs wrtiting here, who come from different sub-cultures, were taught about what an Arab is and also how it was taught (e.g., as history, as a religion, as cultural myth, etc.).
Something I thought would be cool as a work of fiction, would be using a pre-Muslim invasion nationalist who want to restore his original culture and language as the backdrop for a spy thriller – kind of like Day of the Jackal, but set in someplace Egypt.