Israel Folau's church has been labelled an "isolated hate group" by a concerned parent who attended its Bible studies.
The Truth of Jesus Christ Church is run by Folau's father Eni with aid from his cousin, disciple Josiah Folau, and preaches a conservative version of Christianity.
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Israel, a former Wallabies star, is currently enmeshed in a lawsuit against Rugby Australia after claiming he was unfairly dismissed over social media posts saying homosexuals, fornicators, drunks, atheists and others were bound for hell.
After Israel began inviting young players to his church, a concerned parent went along to see what was being preached. What she discovered shocked her, the Sydney Morning Herald reported.
According to the woman, the Folaus told her the Catholic Church was rooted in "heresy, evil and devil worship".
"The Catholic Church is a synagogue of Satan and I 100 percent believe and affirm that Roman Catholicism is masked devil worship," Josiah allegedly wrote to her.
"Any devout Catholic person IS NOT A SAVED CHRISTIAN WHATSOEVER. Look at Catholic doctrine, almost 100 percent of it is false and is filled with lies."
Those not baptised in accordance with the Truth of Jesus Christ Church's rituals aren't considered 'born again' and are going to hell, the church reportedly said. And according to their beliefs, homosexuality is a sin "worthy of death" and women preachers aren't allowed.
"If you believe in women preachers, Satan's got you!" Josiah reportedly said.
"I honestly do not want my son involved in what I have come to understand is false teachings and counterfeit Christianity," the woman told the Sydney Morning Herald. "I've gone, I've checked it out and I would call them an isolated hate group."
It's unlikely to phase the Folaus, who say other interpretations of the Bible are false and those who follow them won't be saved.
"Only we have the truth," the Folaus said, according to the parent.