Haven’t finished brothers Karamazov, but I can comment a little as a former opus dei catholic, now agnostic. According to opus dei theology, every human is born with a built in moral code called Natural Law. This law is perfect, coming from the g man himself, and is the precursor to human laws, which are imperfect after being corrupted by people. Natural law kinda enforces being virtuous, as in the christian idea of virtues (chastity, humility, temperance, etc), as any act that isn’t virtuous is sinful to a degree (here’s where venial and deadly sins come into play, and they mostly rely on the person’s understanding of morality and some arbitrary points). That’s a very simplified rundown, and I might’ve gotten some details wrong, been some years.
Personally I don’t put any stock on any of that. Natural Law by itself is a fine idea, but only because it makes sense. Of course you’ll want to take care of your own and try to make their lives better.