Mr Folau was employed as a tutor at St Gregory's College in Campbelltown, Sydney.
Headmaster Lee McMaster confirmed the decision to 2GB radio, saying: "We have met with Josiah recently and in our discussions, Josiah has made the decision to discontinue his casual employment at St Gregory's College. We respect Josiah's privacy in this matter."
In a letter to a church member, Josiah said: "any devout Catholic person IS NOT A SAVED CHRISTIAN WHATSOEVER.
"Look at Catholic doctrine, almost 100% of it is false and is filled with lies."
"The blasphemous Catholic mass is a paganistic ritual rooted in heresy, evil and devil worship.
He also said: "Roman Catholicism is masked devil worship."
This comes as Israel Folau's legal team begin court proceedings against Rugby Australia and NSW Waratahs.
I confirm I have commenced court proceedings against Rugby Australia and NSW Waratahs. Thank you to those who have supported me over this journey - it has meant so much to Maria and me. God bless. pic.twitter.com/Ojgw9uRw79— Israel Folau (@IzzyFolau) July 31, 2019
The former Wallabies full-back is suing his former bosses after his contract was terminated after he posted "hell awaits drunks, homosexuals, adulterers, liars, fornicators, thieves, atheists, idolaters" on his Instagram account.
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