separatist

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

• noun One who secedes or advocates separation from a group, as from a nation or an established church.
• noun One who advocates cultural, ethnic, or racial separation.

from The Century Dictionary.

• noun One who withdraws or separates himself; one who favors separation. Especially
• noun In recent British politics, an epithet applied by the Unionist party to their opponents, whom they charge with favoring the separation of Ireland from the United Kingdom.
• Of, pertaining to, or characteristic of separatists or separatism; advocating separation: as, separatist politics; separatist candidates for Parliament; a separatist movement.

from the GNU version of the Collaborative International Dictionary of English.

• noun One who withdraws or separates himself; especially, one who withdraws from a church to which he has belonged; a seceder from an established church; a dissenter; a nonconformist; a schismatic; a sectary.

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

• noun Someone who advocates separation from the established Church; a member of any of various sects or schismatics.
• noun A person who advocates or seeks the splitting of one country or territory into two politically independent countries or territories.
• adjective Advocating ecclesiastical separation.
• adjective Advocating or seeking the separation of one country or territory into two politically independent countries or territories.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• adjective having separated or advocating separation from another entity or policy or attitude
• noun an advocate of secession or separation from a larger group (such as an established church or a national union)