It's... at least the official doctrine of most Christian churches, yeah, and that's usually held as a particularly important point. Catholics, certainly, and they're rather numerous. It is possible that many, or even most, Christians disagree with tenets of faith that their churches hold to be absolutely vital and indispensable, I suppose.
[EDIT] It's part of the Nicene Creed, even. I think it's fair to say that if most church-goers at most Christian churches told a priest or pastor that they didn't believe that God miracle'd a baby into the Virgin Mary, it would, at the very least, give the priest or pastor a frowny face and/or evoke a "wait... what exactly do you think we're up to, here?" reaction.
[EDIT] It's part of the Nicene Creed, even. I think it's fair to say that if most church-goers at most Christian churches told a priest or pastor that they didn't believe that God miracle'd a baby into the Virgin Mary, it would, at the very least, give the priest or pastor a frowny face and/or evoke a "wait... what exactly do you think we're up to, here?" reaction.