Firstly install Notepad++ and open the file infernax.666
This file contains game related settings and options.666 have sound etc settings.
Search moralityScore in this text and set it to 34 to unlock "Protect the relic quest".
Warning: changing the length of file will corrupt the save file. Make a backup first!
You can get Whatever XP upto 9999. Search "xp": and change the 4 digits only. DO NOT ADD extra characters to change total length of save file. If morality is 2 digit number then don't make it 3 digit. Same applies to other settings. This way game can be hacked to get your quests or enable hidden locations etc.
Since save state is single any bad decision will ruin the game and replay is needed therefore backing up of save is important. Or trick with morality score and other settings.
How to get full demonology by hacking save file?
Just replace "numKilled":0 to "numKilled":1
This means all demons will be set to be killed at least once. This will give full demonology by hack!. This will enable Gregor to send you to future. Ultimate weapon is then granted to kill the final boss.