By the 17th century, especially the 1640s when the film is set, the only remaining practicing Christians were likely to be die-hards. Much of the behavior shown among the converts in the film and novel — reluctance on pain of death to disrespect Christian images or to express verbal contempt for the Virgin Mary, placing urgent importance on baptism and confession, the convert Monica’s belief that Heaven will be superior to their current state because it removes them from the suffering of feudalism — is pretty orthodox for a faithful 17th century Catholic, even one born and raised in Europe. The priests are their means of accessing the sacraments (besides baptism, which iirc is handled by the religious community’s layperson leader, the jiisama) and engaging with what they see as their own salvation and that of their children in an isolated and politically oppressive context that restricts their access to Christian images as well as prayer. Furthermore, they’re Catholic in the first place — worship using images and songs isn’t unique to Japanese Catholics but in the context of the film these traditional elements are guarded so fervently and jealously because they’re left over from an earlier era of comparative religious toleration.

However, not all Japanese people in this era who had formally received Christianity via baptism cleaved to it to that degree — in addition to those who renounced Christianity under duress there were others who left off Christianity willingly, either for political expedience after the ebb of direct Catholic influence in Japan or out of general waning interest. (The film’s version of Inoue is in one of those latter categories.) After severing contact altogether with external European Catholic influence, remaining Japanese Christians (“hidden Christians”) ultimately developed their own customs and worship styles independent of Roman Catholicism and independent of the services of European-style priests. I’m on mobile right now so I can’t do a preliminary roundup of sources but I’d love to give any information that might help contextualize why the film’s Japanese Christians appear so fervent and why their reliance on the two European Jesuits in their midst is so intense.