From wikipedia “Most divisions of Christianity are Trinitarian, and regard belief in the Trinity as a test of Christian orthodoxy.” As it turns out the only familiar branches of Christianity which do not accept the trinity are Mormons and Unitarians, which is the main reason why many don’t accept them as Christian. In ancient times the Arian and Monophysite heresies didn’t accept the Trinity, which is why Islam was accepted by many formerly Monophysite and Arian Christians.
I doubt anyone accepts islam unless conquered by islamic peoples.