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The Church of Scientology is on the attack against the director of a documentary eviscerating the org., claiming the guy who made the most indicting claims is a known liar.

A rep from Scientology tells TMZ ... the man featured in the movie who claims Church officials illegally wiretapped Nicole Kidman cannot be believed because he has admitted under oath in a court case he's lied about the Church before.

The rep says in December Marty Rathbun -- a former Scientology exec featured in the documentary -- was under oath

late last year and admitted he lied about Scientology practices. He also admitted to obstruction of justice.

We've confirmed Rathbun did indeed lie, but his position -- which he discussed in the deposition -- is that he lied to protect the Church and cover up its misdeeds. Rathbun says he was an executive in the Church at the time and did not want to squeal on alleged misdeeds of Church leaders.

Rathbun claims in the documentary that he was tasked with facilitating the breakup of Tom Cruise and Nicole Kidman, because her dad was a psychologist and the Church felt he was influencing the actor into distancing himself from Scientology.