Islam isn't an offshoot of Christianity. Christianity is a Protestant Reformation version derived from Christendom--another name for Catholicism. Islam is something almost independent of Catholic influences. Those that believe the two are the same don't understand either religion.
Christians hold the Bible as the only religious textual authority; Muslims hold the Quran as such. Both think the others' holy text is flawed. Christians revere Jesus as God; Muslims do not. Christians call their God YHWH meaning "ever-existing" "eternal one" or "to be (forever)"; Muslims call their God Allah meaning "the God" (God basically means "supreme being"). The names aren't the same (regardless if descriptions of the being as similar). The Bible is 2/3rds of prophecy, the Quran is 1/6ths of prophecy. Revelation claims no other revelatory text will come from God. Muslims claim the Quran is a revelatory text from God.
Islam isn't an offshoot of Christianity. Mormonism is. Jehovah's Witnesses are. Not Islam.