From: Rabbi Akiva Eiger
To: Unknown recipient
Rabbi Akiva Eiger resolves why the majority principle (rov metzuyin mumchin hein) does not apply to a mumar performing shechitah. He demonstrates that for a mumar the minority of non-experts constitutes a regular mi'ut rather than a mi'ut d'mi'ut, and since halacha accepts smoch mi'uta l'chazaka per Rava, this minority combines with the chazaka of eino zavuach to require verified expertise. This shows the Rambam's ruling is grounded directly in the Gemara, not a novel invention.
Source: Shu"t Rabbi Akiva Eiger §163