Review Detail of Luninu in I Became The Pope, Now What?

I have some issues with this Novel - that is, from the chapters 500 and later. Before, the novel is pretty neat and can be read without any major problems in the plot. But even then, somethings are really explained poorly: mainly the power and cultivation system. I still have NO clue how the mc cultivates and becomes stronger, literally, he seems to just do it. (I did ask the Author privately and he admitted to it, but the plot and story progress, which was in the focus, has given me much fun) The larger plot 'twist' at around ch500~600 is pretty bad and has given me a bad taste, also being the reason why I stopped reading it. I do not comprehend the reason to do what the author did with the mc, but I could guess he tried to make the mc pope in a quick yet critical situation. Nonetheless, at least till ch600, it's a very interesting and good read.