I have tried to read the "blessed be Kerah" on the medallion but nothing happend.
It says in the walkthrough that this will open the "MAIN CRYPT"
Can someone please help me?

You have to speak to the statue. Type in those words, "Blessed be Kerar", and the statue will answer you with something like, "Only the faithful may follow". That's when the passage behind you opens. Hope this helps and good luck! Save often!!!