Israel Folau's cousin Josiah Folau has been let go from his casual teaching job at a prestigious Catholic school in Sydney's south-west after he described the Catholic Church as "the synagogue of Satan".

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 Herald revealed on July 20 Josiah's involvement in the 30-member Jesus Christ Church founded by his uncle, Israel Folau's father, Pastor Eni, in Kenthurst.

The church preaches that any Christian who is not "born again" will go to hell.