An emotional two-page letter written by a sexual abuse victim has been credited with helping convict an Australian Archbishop of concealing such crimes dating back decades.

Adelaide Archbishop Philip Wilson stood down on Friday after he was convicted for concealing child sex abuse at Newcastle Local Court. He is the highest-ranking Catholic official in the world to be convicted of covering up abuse in the church.

The letter which helped bring his case before a court was penned by Peter Creigh, who was abused by paedophile priest James Fletcher more than 50 years ago.

Mr Creigh detailed his harrowing childhood as an altar boy in the two-page note written in 2010.

'I had a choice. If I didn't tell them it would have got to me and I would have become another statistic,' Mr Creigh told The Advertiser after Wilson's conviction.

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A two-page letter written in 2010 by a victim was all it took to take out an Australian Archbishop Philip Wilson (pictured) who was convicted for concealing child sexual abuse

Letter (pictured) written by Archbishop Wilson after he was convicted in Newcastle Local Court

In the letter, Peter Creigh (centre) details his harrowing childhood as an altar boy who was abused by priest James Fletcher

Mr Creigh said the sexual assaults occurred in 1971 at the hands of Father Fletcher.

In the letter, Mr Creigh explosively alleged he had told Wilson of the abuse he endured in 1976.

The letter dated July 29, 2010, was found by the Special Commission of Inquiry in 2013 after being kept secret for three years.

Mr Creigh concluded: 'For us victims of sexual abuse our lives will not be the same again, we have been mentally, and in some cases physically scarred for life — that is the sad part.'

The Catholic Archbishop was found guilty of covering up father Fletcher's sexual abuse of altar boys from the 1970s on Tuesday.

Abuse victim Peter Creigh said he went to see Father Wilson at the presbytery of St Joseph's Catholic Church (pictured) in East Maitland in 1976 to tell him about what Fletcher had done to him five years earlier

Wilson was found guilty of covering up father Fletcher's sexual abuse of altar boys from the 1970s on Tuesday. Prosecutor Gareth Harrison said Wilson was involved in a cover-up to protect the church's reputation

Prosecutor Gareth Harrison said Wilson was involved in a cover-up to protect the church's reputation — arguing that child sex abuse 'victims came second' in Wilson's mind.

'Archbishop Philip Wilson has today been found guilty of failing to inform police about allegations of child sexual abuse,' Archbishop Mark Coleridge said on the Catholic Archdiocese of Adelaide's site.

'Archbishop Wilson maintained his innocence throughout this long judicial process. It is not yet clear if he will appeal the verdict.'

Wilson officially stepped aside from his role as Archbishop on Friday.