The case is France's most important church sex abuse case to date and poses a new challenge to the Vatican, amid growing demands in overwhelmingly Catholic France for a reckoning with decades of sexual abuse by the clergy.
Cardinal Philippe Barbarin, 68, appeared in a Lyon (pictured below) court along with other senior church officials accused of failing to protect children from alleged abuse by the Rev Bernard Preynat.
The top Vatican official in charge of sex abuse cases, Cardinal Luis Ladaria, is among the accused - but will not appear in court because the Vatican invoked his diplomatic immunity.
After taking his seat in the front row, Barbarin held his hands together and closed his eyes, as if in prayer.
The judge then read out the lengthy accusations against the defendants, and the defence used procedural arguments to try to get the case thrown out.
Nine people who said Preynat abused them in the 1970s and 1980s brought the case to court, and hope it marks a turning point in efforts to hold the French church hierarchy accountable for hushing up abuse.
The victims say top clergy were aware of Preynat's actions for years, but allowed him to be in contact with children until his 2015 retirement.
Despite nationwide attention on the case, it may fall apart for legal reasons.
Prosecutors initially threw it out for insufficient evidence, and Barbarin maintains his innocence.
His lawyer says his client never obstructed justice because the statute of limitations had passed on the acts in question by the time Barbarin was informed.
"Accusing an innocent man doesn't advance a cause," lawyer Jean-Felix Luciani said.
If found guilty of failing to report the priest's actions, the defendants could face up to three years in prison and a 45,000 euro (£40,000) fine.
Barbarin and some other defendants are also charged with failing to assist a person in peril.
The case is a new test for Pope Francis (pictured below), whose blind spot on clergy sex abuse has threatened his legacy and thrown the Catholic hierarchy into a credibility crisis.
Francis has praised Barbarin as "brave" and said French justice should take its course.
Preynat, now in his seventies, wrote letters to some families confessing the abuse, and is to be tried separately on sexual violence charges involving 10 children.
One of his alleged victims, Alexandre Hezez, hailed the trial as an effort to "move justice forward".
Mr Hezez, 44, spoke to the cardinal directly about Preynat and is among those who brought the case to trial.
Barbarin sought counsel on how to handle abuse accusations against Preynat from Cardinal Ladaria, who recommended disciplinary measures while "avoiding a public scandal".
Numerous child sex abuse claims have been made against Catholic clergy in France since the 1990s, but there has not been a huge wave like those seen in the US, Ireland or some other countries.
Barbarin is the highest-level French church figure accused of covering up abuse, and his case has cast a shadow over the diocese and the French Catholic Church.
As a result, the Bishops of France last year created an ambitious commission aimed at shedding light on sexual abuse of minors in the church since 1950.
A report is due in 2020, but the issue is divisive.
An outspoken French priest, the Rev Pierre Vignon, started a petition urging Barbarin to resign that garnered more than 100,000 signatures last year - and Mr Vignon says the effort has damaged his church career.
Barbarin is among the most powerful figures in the Catholic Church, one of some 200 cardinals worldwide and archbishop of Lyon since 2002.
Multiple cardinals have been accused in recent years of shielding abusers or committing abuse themselves, from Pennsylvania to Australia to Chile.
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