Talk to Priest Zigaunt. You must prove your pure faith by removing all inappropriate books that criticize the current order. The first rumors about those books appeared in Gludio. Go and investigate the book, Understanding of Miracles. He gives you a Letter of Order.
Go to Gludio and talk to Trader Simplon.
Talk to Guard Praga and the other by Priestess Vivyan about the other two books.
Kill Ruin Zombies just outside Ruins of Sorrow/Agony to get Pendant.
Return to Praga for book.
Return to Priest Zigaunt.
Find Lionel on Talking Island, near the road exiting the Elven Ruins, and deliver letter.
Take book to Grand Magister Gallint.
Return to Lionel and get the Covenant.
When you are Level 20, return to Gludin and Speak with Priest Zigaunt to become a Cleric.