Mr Folau worked as a tutor and boarding house supervisor at St Gregory's College at Campbelltown. He was also an alumnus of the school, who finished eighth in the state in religious studies in the 2016 Higher School Certificate.
A staff member confirmed the decision on Thursday. The school's decision came after the Sydney Morning Herald revealed Josiah's involvement in the 30-member Jesus Christ Church founded by his uncle, Israel Folau's father, Pastor Eni.
The church preaches that any Christian who is not "born again" will go to hell.
"Look at Catholic doctrine, almost 100% of it is false and is filled with lies," Josiah wrote.
"The blasphemous Catholic mass is a paganistic ritual rooted in heresy, evil and devil worship.
"Roman Catholicism is masked devil worship," he also said.
The parting of ways between Josiah Folau and St Gregory's comes as Israel Folau's legal team formally filed a claim of unlawful termination against Rugby Australia in the Federal Circuit Court in Melbourne on Thursday.
Folau is claiming $10 million in damages following his sacking by RA in April over an Instagram posting in which he paraphrased a Bible passage, saying "drunks, homosexuals, adulterers, liars, fornicators, thieves, atheists and idolaters" would go to hell unless they repented.
The legal decision comes after the former Wallaby and RA failed to reach an agreement at a conciliation hearing at the Fair Work Commission on June 28.
"Unfortunately, our conciliation before the Fair Work Commission did not resolve the matters between us and I have been left with no choice but to commence court action," Folau said in a statement on Thursday.