Res Publica Christiana
Res Publica Christiana is an Outer Colonies faction that is composed of various conservative and national democracies (that endorse an important role of religion and ethics in both society and politics), and monarchies in which monarchs are bound by constitutions and/or parliaments. What is a common denominator is the Neocatholic Faith, in billions of its internal variants, with different liturgies and doctrinal variants. They are banded together with bonds of religion and relatively similar internal policies, including a support for personal rights, market liberal or ordoliberal economy rights, and utter disdain directed at various radicals, including Pact of Steel, Neocomintern and Fifth International.
A love/hate relationship.
While NNAT states in most cases aren't really all that religious, they are still one of two possible allies for RPC (the second being Ancien Regime), and the one more likely at that. As such, they are force to cooperate against extremists, even though they aren't overly lovey dovey with eachother.
Both sides hate each other, although the Res Publica Christiana reserve its deepest hatred to Pact of Steel. Fifth International hides its revolutionarism and hatred well.