Some Muslim groups believe everybody is born Muslim. Some don’t.
The Quran basically teaches that everybody in the Bible was a pre-Muhammad Muslim, but that Jews and Christians faked up their own version to remove the Muslim parts (and the part where everybody lived in Arabia). So you get a situation where Jewish saints and Christian martyrs are rewritten as having been Muslims. And in such a case, obviously Christians and Jews are just Muslims with the wrong ideas.
The problem, of course, is that a Muslim apostate who refuses to come back is under sentence of death, as commanded in the Quran. So thinking of Jews and Christians as actually Muslims – it is not a happy ecumenical thought to have. It is hostile in the extreme.
The other consequence is that, according to Muslim law, any group with a religious book and some kind of monotheism is supposed to be treated more favorably than pagans. However, this doesn’t apply if you’re an apostate.
On the opposite side, some Muslims like to argue that Christians are polytheists (because of Jesus and the Holy Spirit), and that therefore they don’t get People of the Book protections.
Individual Muslims can be people of good will, with honest feelings toward Christians. But the system of Islamic religion and law is generally hostile toward everybody who’s not a Muslim (and some who are).