In the Castlevania universe, to me, it seems that christianity is treated with the same respect all other religions/beliefs are. It's an universe where "all legends and myths are true", including gods and demons.
The Belmonts may wield crosses and traditionaly christian symbols to fight the forces of darkness, but maybe because THEY are christian, and not because "only christianity works". The symbols they use may have effects against evil not because they're "christian", but because they Belmonts place faith in them (not to mention that these symbols are weaponized, too). There are OTHER holy symbols from other beliefs in the Castlevania universe that also carry power against the darkness, leading me to say, again, that it's about faith you put into it and not about the belief the symbol comes from.
God, in the series, seems much like an abstraction of good, and not like " THE Abrahamic God". A
force of good, not an
personal entity of good.
Dracula himself in the series is analogous to Satan. He's THE force of evil, opposing God (As Alucard puts and Dracula's own title of "Mao" suggests). However, his character does not belong to any belief. Dracula is simply the Dark Lord, harbinger of chaos and evil, and ultimate manifestation of this evil on Earth.
There are other gods in the series (Maria's summons, Juste summons, creatures serving under Dracula that are clearly evil gods or pagan gods). That's why I say that there is no "Abrahamic God" per se. They seem to treat "god" as the overall force of good, and under this force there are many good entities and gods.
Sure, Castlevania started with heavy christian ideology (thanks to the vampire myths about crosses and holy-waters), but it seems to me that it has evolved beyond "christian" to encompass a plethora of beliefs about good and evil exactly to express a
battle of good and evil, and not just "christian against vampire".
As Nagumo put, the ultimate technique to rid the world of Dracula was not even christian: It concerned beliefs about Amaterasu and Susanoo and their relation with the eclipse.
Also, this:
Given Alucard's use of holy water, Bible, and cross within his armament, it doesn't seem to fall in line with what he says in his ending. There doesn't really appear to be anything that suggests a pluralistic world, i.e., the Christian God exists alongside other gods in the other games.
You are ignoring the fact that Alucard also uses a Vibhuti (holy ashes from hindu belief) and the Agunea (holy lightning slavic mythology), both coming from different beliefs but also have effects against evil. Alucard also has a range of weapons coming from different belief systems. Thunderbrand, for instance, is called "Indra's Lightning Sword" in japanese. A bunch of other weapons/items suggest the presence of other gods/entities, but not only that, Maria has access to four
incredibly powerful chinese gods.
SotN was not so mixed about beliefs, true, but this was thrown out the window with subsequent releases.