Cardinal George Pell is the most senior Catholic to be convicted of child sex abuse and is appealing those guilty verdicts in the Victoria state Court of Appeal.
Prosecutor Chris Boyce told the three judges on Thursday that the convictions largely based on the testimony of a single accuser should stand.
"The complainant was a very compelling witness," he said.
"He was clearly not a liar, not a fantasist."
A jury unanimously convicted Pope Francis' former finance minister in December of orally raping a 13-year-old choirboy and indecently dealing with the boy and the boy's 13-year-old friend in Melbourne's
St Patrick's Cathedral in December 1996.
Pell had become archbishop of Melbourne only months before.
One of the boys, now in his 30's, is the key prosecution witness.
The other died or a heroin overdose in 2014, apparently without ever alleging he had been abused.
Pell was also convicted of an act of indecency against the complainant by squeezing his genitals as they passed in a cathedral corridor in early 1997.
Mr Boyce said the complainant spent more than eight hours in the witness box and underwent "searing" cross examination during Pell's original trial in the Victoria County Court that ended with a deadlocked jury in September last year.
"If it was a fantasy, at some point you'd expect the cracks to appear," he said.
A second jury convicted Pell after watching a video of the complainant's testimony. The appeal judges will also watch the same testimony.
Mr Boyce told the judges they need not consider why at least two jurors in Pell's original trial were not convinced beyond reasonable doubt of his guilt.
Pell was sentenced in March to six years in prison.
He is held in special protective custody because paedophiles are regarded as being at higher risk of harm from other prisoners.
Pell's lawyer Bret Walker had told the court on Wednesday the cardinal could not have molested the two boys and argued the five verdicts against Pell were "unsafe and unsatisfactory" and should be overturned.
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