The following page of our guide to Greedfall contains a walkthrough for Face to Face with a God main quest.
Talk to the newly elected king about the promise they made before the election. Then talk with the High King to open the gates of the sanctuary for you. Enter and, as suggested by the clan leader, keep to the right side of the area at all times.
The path will not be too long, and after a while get to the cave, where you will find a wide tree. This is En on mil frichtimen, God of the island, which will give you very important information and warn you of the evil plans of Constantin. After the cutscene, a monster will emerge, and the conversation will end.
Leave the Sanctuary the way you came to En on mil frichtimen. Coming out of the sanctuary, you will encounter the High King, who will want to talk to you - once the conversation is over, the quest will be completed. You will get 2000 XP and you will have to go to New Serene to talk to Constantin.
During the conversation with Constantin, you learn that San Matheus and Hikmet need help. You will get two quests -Attack on Hikmet, and The attack on San Matheus, which you can do in any order.