So the official name for the child of Maria and Mamoru is 救世主 (Kyūseishu), which basically means "savior" or "the Messiah"... It is unusual (and ironic) for a character to be called like that -- especially when she(he?) is pretty much the biggest threat for humanity at that time. Why Messiah among all names? Is there a reason for this?
It's because Messiah is the savior or the Messiah for the Queer Rats, not the humans.
It's implied that Squealer found out (through a False Minoshiro) that the Queer Rats
are actually genetically altered humans (mixed with the DNA of mole rats as Saki speculates at the end)
and for all intensive purposes, slaves to cantus users much like that of the Slave Dynasties (where non-cantus humans were slaves to the ruling class of cantus users). Any attempt of rebellion against the humans would be slaughter because the Queer Rats can't fight against cantus powers in the end.
Supposedly from the same source Squealer learned about the Death Feedback (and possibly what an Orge is) in which cantus-users of the 4th group have genetically altered themselves to be unable to kill other members of their own speices. we even see earlier in the series that something that looks human but isn't (like a Queer Rat in the distance) can trigger the adverse reactions of the Death Feedback.
By raising Maria and Mamoru's child as a Queer Rat (via imprinting) he/she (in the anime it's a girl, in the manga it's a boy, not sure about the novel) would not be affected by the Death Feedback when killing humans (because to Messiah he/she is a Queer Rat or that Queer Rats are human and actual humans are something else) and as described by Tomiko Asahina when she was telling Saki about when the village was in danger from an Ogre it took deceit and mutual destruction to kill a single Ogre (the doctor who killed the boy who was the Ogre ended up dying). This is even more evident when Shisei Kaburagi faced against Messiah where despite being the stronger cantus user he was weakened and killed by Messiah.
Now to my understanding Messiah as a name is never used by the humans and it's the Queer Rats, mainly Squealer, that refer to Messiah as their Messiah, their savior. Most of the humans refer to him/her as an Ogre while Saki refer her to as Maria's (and Mamoru's) Son/Daughter