Mark Rathbun is a former Scientologist who once worked close to the ecclesiastical leader of the Church of Scientology, David Miscavige. After revelation of gross misconduct, violence against other and alterations of Scientology he was demoted and left the church in 2004. Since about 2009 has publicly been an anti-scientologist and a squirrel.
During his time in Scientology Mark Rathbun called himself "Marty" Rathbun.
In 2009 and 2010 Mark Rathbun continued his public confessions (this time in the media*). He said he tried to damage the Church of Scientology and its leader by destruction of court evidence he was responsible for, and physically violent conduct towards others. He also publicly bragged with telling others to lie about their Church, in an attempt to "win" a legal case (it took Church leader David Miscavige to reveal this plot, straighten out the case and get everyone involved to apologize to the court).
In May 2010 Rathbun also said he sabotaged an internal correction system for auditors - the "look in system", where counseling sessions are videoed by visible cameras directed on the e-meter and the room and with the agreement of all concerned, for review by the Case Supervisor whose task is to ensure the positive progress of the session, so that his misapplication of Scientology would not be detected for a long time.
Rathbun's misdeeds were severe violations of the Church of Scientology's ethical codes which by themselves, warrant immediate expulsion from the Church.
There are rumors that Mark Rathbun is trying to take over the Church of Scientology or that he is part of some schism of Scientology, which is at odds with the fact the international network of Scientology churches has seen its biggest expansion since Mark Rathbun left. No surprise for Scientologists and easily explained: the continuous destructive actions of Mark Rathbun during his time as church staffer were removed together with him. More information about the growth of the Church of Scientology here.
*In summer 2009 the Church of Scientology published several transcripts dealing with Mark Rathbun's confessions to the media. It is online here:
Note: There is a prominent sociopath with the name Mark Rathbun (who was sentenced in December 2008 to over 1,000 years of prison for rape and sexual assault on 67 occasions). Though one could think of Mark "Marty" Rathbun as some kind of "psycho", these two Mark Rathbun's are not the same person (http://whois-markrathbun.com/).