High Priest Sylvain:
You need the appropriate level and token to become a Cleric. To change profession, your level must be at least 20, and to become a Cleric, you must have the token that proves you have passed the Test of the Cleric.
You have achieved a sufficient level, but it seems you have not yet passed the Test of the Cleric. You will be acknowledged as a true Cleric only when you have passed the test. If you wish to take the test, visit Priest Zigaunt in the village of Gludin. You must pass his test and bring back the mark of faith to change profession to Cleric. As I'm sure you know, just follow the road west to reach the village of Gludin.