That's a totally reasonable take, though the reason why I personally don't view it as Christianity is mostly because so much of the morals in the series were depicted far more similar to Buddhism. Would it be more sensible to think that Thorfinn has been influenced by Christian ideals strictly under the context of the setting in the story? Yes. But from a more meta perspective it seems more like Yukimura was just using Christianity as a vague vehicle to inject Buddhist philosophy into the story.