The George Pell case was always going to be a flashpoint.

Cardinal Pell has already said his case was never about the Catholic Church's response to child sex abuse.

But there are many who saw it as exactly that, and have been watching on with close interest.

For his supporters who have been equally engrossed in the case, Tuesday was a vindication of their belief in his innocence.

The key question was whether it was open to the jury to arrive at a guilty verdict or whether the evidence provided reasonable doubt about the Cardinal's guilt.

On Tuesday, after a journey through the Victorian criminal courts and the Court of Appeal, the High Court found there was room for reasonable doubt and Cardinal Pell is now free.

Cardinal Pell faced two juries on child sexual abuse charges involving two choirboys in the sacristy at St Patrick's Cathedral when he was archbishop of Melbourne in the late 1990s.

One of the boys had died by the time of the prosecution, so Cardinal Pell was convicted on the evidence of the other.

Cardinal Pell never gave evidence, but when he was interviewed by police he vehemently denied the allegations.

The High Court found the Court of Appeal had made a subjective assessment about the complainant's truthfulness. ( ABC News: Gregory Nelson )

The first jury could not reach a verdict, but the second found him guilty.

That was later backed up a Victorian Court of Appeal ruling.

To unpack why the High Court overturned both these findings, it is necessary to look to the evidence in the case.

The question of reasonable doubt

The central question in the case was whether it was open to the jury to find George Pell guilty, or whether the jury should have had a reasonable doubt after hearing the evidence.

The prosecution relied entirely on the truthfulness and reliability of the complainant's evidence.

That was upheld by the Victorian Court of Appeal, which described him as a witness of truth.

The defence relied on evidence from people who worked in and around the cathedral, whose evidence suggested the opportunity for the alleged offences was unlikely.

This included then-archbishop Pell's practice of standing on the steps outside the cathedral after mass, the tradition that he would never be alone while robed, and the hive of activity in the area near where the alleged offences were said to have happened.

Indeed, in the original trial the defence had described the evidence as showing the alleged offences were impossible.

But by the time the case reached the High Court the language had softened to improbable.

Witness vs 'opportunity evidence'

Cardinal Pell's lawyer set out a cogent case to explain why the jury and the Court of Appeal should have found that the "opportunity evidence'" suggested there should be reasonable doubt about his guilt.

On Tuesday the High Court agreed.

In its ruling, it took aim at the Appeal Court judgment, saying the majority had made a subjective assessment of the truthfulness of the alleged victim.

"[That] drove their analysis of the consistency and cogency of his evidence," the High Court judgement said.

The High Court said that was at the expense of defence evidence which suggested there could be reasonable doubt about his account.

"The analysis failed to engage with whether, against this body of evidence, it was reasonably possible that A's account was not correct, such that there was a reasonable doubt about the applicant's guilt," the court said.

Cardinal Pell's lawyers maintained there was reasonable doubt about his guilt because of the "opportunity" evidence. ( ABC News: Danielle Bonica )

The conduct of appeals has long been in the High Court domain.

Rules suggesting appeal courts are to make an independent assessment of the evidence and then decide if it was open to the jury to arrive at its verdict, now known as the M test, were established in a 1994 High Court case.

One question in the George Pell case was whether the Court of Appeal had gone too far in its assessment by viewing video evidence from the case, or whether it should have stuck to the transcripts.

On Tuesday the High Court ruled that videos were allowed, but only in exceptional circumstances.

The hearing was run over two days.

Things did not go well for the Victorian Director of Public Prosecutions Kerri Judd, who found herself on the wrong end of several exchanges with the judges, particularly when she shifted ground on whether the alleged incident was longer than five or six minutes, something not put during any earlier hearing.

On Tuesday the High Court ruled unanimously Cardinal Pell's guilt was not established to the requisite level.

The appeal was allowed, the convictions quashed and George Pell was acquitted.

Read the High Court summary:

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