true believer

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One who is deeply, sometimes fanatically devoted to a cause, organization, or person.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun ecclesiastical A strict follower of a religious doctrine.
• noun idiomatic One who sticks to one's dogma or beliefs irrespective of the facts (see true believer syndrome).