You can change jobs to a Priest in the Sanctuary of Prontera. Apply to Father Thomas, in the Sanctuary located in the 2 o'clock direction of town. If you pass each level of training, you will be able to change jobs to a Priest. The test consists of spiritual trainign to test your strong belief, and physical training. After the test is completed, sign an oath with Sister Cecile and talk to Father Thomas to become a Priest. Requirement are Job Lv 40 or higher.