After attempting to rescue a prisoner of the Cast, you search for Bishop Nelson's cult.
Stop the Cult
In an area west of Goodheart Castle, you will find a couple of torches with a warning sign nearby saying to keep out. You will be injured if you try to walk between those torches, so don't. Instead, equip the magical scroll and use it to say a magic spell. The barrier between the torches will be destroyed and you can walk up through them.
There are many enemies in this area. You can skip them for now if you want. Just go all the way to the right, then all the way north, then west. You will see Bishop Nelson's cult at work, and he will bring Nadine's daughter Mary as the newest cult maiden. Choose the "say something..." option when it comes up.
Dink will intervene. The girls transform into Boncas. Kill them, and then Bishop Nelson will transform into a Slayer. Kill it. It moves really slowly, so just keep your distance and use fireball magic until it's dead.
After the monsters are all dead, talk to Mary and tell her that you are a friend of the land. You will take Mary home to Nadine. After that, be sure to talk to Nadine and ask how she is doing.
Help the Suspicious Guy
Now go to the bar and talk to the guy at the bottom right. (If he isn't there, talk to Nadine and ask how she is doing.) Ask what the guy does, then talk to him again and offer to help him out. Go to the screen with Nadine's house in it. The guy from the bar will be there and you will go into the locked door with him. Then some guards will attack. To make it easier for yourself, make the guards walk close together, then go close to them. They should end up attacking each other until one of them dies. That leaves you with only one guard to kill. When they are both dead, go outside.
KernSin is now open, so follow the dirt path south from Nadine's house, then continue following the dirt path to the west to get to KernSin.