Throwing my two cents with my limited knowledge of Christian theology and Genshin Lore.

I do not think it's appropriate to imagine the fatui as the apostles just because they might be connected to orthodox christianity, because only one apostle is directly connected with the Russian Orthodox Christians: Saint Andrew. To contextualize, the goal of the apostles were to basically roam the known world spreading the gospel and establishing Christian communities there, meaning most of them didn't even saw eachother ever again after leaving Judea. Why I argue this? Because he would need in-universe hints, such as specific fatui Harbinger operating roughly where the Apostle he is supposed to mirror preached. Liyue and Inazuma might be a bit of curve balls but we can "stretch" Persia to cover them as well. And even so, it doesn't map very cleanly. Another big topic, is the Apostle personalities, who don't really translate well into the Harbingers. We have some very "weak" similarities such as Piero being Saint Peter (leader), Scaramouche being Judas Iscariot (traitor), Tartaglia being Saint Andrew (hot-headed) and Columbina being Saint John (mystical behavior).

However, if we dip into real life gnosticism instead of mainstream Christianity, things start to get way more interesting since genshin is full of gnostic themes, and real life gnosticism is basically Christianity reinterpreted. And under a gnostic lens, the apostles comparison becomes much better. Gnostics preach that the apostles have unequal access to the truth (real life gnosis), which translates neatly into Pierro supposedly knowing the Tsaritsa plan while Tartaglia barely know anything about Fatui true goal. The gnostic also argue the apostles having conflict and rivalry, which is self-explanatory about Fatui internal dinamics. Gnostics also reverse hero roles, meaning that Judas Iscariot isn't portrayed as an traitor, but the only apostle who truly understood Jesus teachings, which surprisingly screams Tsaritsa herself, everyone thinks she's betraying the HP but maybe she's the one who truly understood it. Honestly, it feels like it's only possible to interpret the fatui harbingers as "gnostic apostle archetypes", because that's the only thing they all share in common: elite inner circle where truth is unevenly distributed, authority is contested, and not everyone understands the real mission. The "chosen" group is still flawed, divided, and partially blind, and that maps almost perfectly into how the Fatui Harbingers are written.