abjurer

to renounce or abandon solemnly; to abjure; to formally renounce one's religious belief; to apostatise; to reject by oath someone's authority

From Middle French abiurer, abjurer, from Old French abjurer (“to reject or renounce (something) on oath”), borrowed from Latin abiūrāre (“to deny on oath, recant, renounce, repudiate, abjure”).