This chapter of the guide to GreedFall has the walkthrough for Trouble in Eden side quest.
Where to start the quest?: Speak with the Mother Cardinal in San Matheus during, for example, The Vices of the Mother Cardinal side quest.
Go to Tir Dob. "The Black Lands" and talk with Father Lustinius. He will tell you the story of Eden - once he is done, ask the theologians standing next to him about the missing St. Matheus tablets. Go to the cabin and investigate the place where the theft took place.
Talk with Ler, the former leader of the village and with Vidwal standing a little bit further. You will learn where to find the tablets - go there, and then talk to Bran. You can convince him using charisma to return the tablets, ask if there is anything you can do for him to return them, or threaten him.
Eventually, you will get the tablets - go back to Eden. For their recovery, you will get + 2 Theleme reputation points with and 1,300 XP. You will also start Theological Conflicts side quest which can be done later, once you complete more main missions.