Cardinal Joseph Zen holds a Mass in Hong Kong on May 24, after he was released on bail.

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Cardinal Joseph Zen, a 90-year-old former bishop of Hong Kong and outspoken critic of China’s ruling Communist Party, went on trial Monday over his role in a relief fund for the city’s pro-democracy protests in 2019.

The high-profile case has brought renewed focus on the warming ties between Beijing and the Vatican, which has seen the latter appear to avoid remarks that could risk upsetting China.

Zen, one of Asia’s most senior Catholic clerics, was arrested by Hong Kong’s national security police in May along with three other leading democracy activists, including Cantopop star Denise Ho.

The four trustees of the fund were initially arrested on suspicion of “collusion with foreign forces,” a charge under a sweeping national security law that carries a maximum punishment of life imprisonment.

A fifth trustee, former legislator Cyd Ho, was already in jail for illegal assembly over a separate case. Ho was arrested the following day on national security grounds, too.

All five have since been charged with a lesser offense for failing to register the 612 Humanitarian Relief Fund, set up in June 2019 to help pay the legal and medical fees for protesters under arrest. The fund ceased operations last year following an investigation by the national security police.

The charge under the Societies Ordinance, a century-old colonial-era law, carries a fine of up to HK$10,000 ($1,274) but not jail time. All defendants have pleaded not guilty.

On Monday morning, Zen arrived at the West Kowloon court leaning on a walking stick. He did not speak to reporters.

The court heard that the legal fund raised HK$270 million ($34.4 million) of donations, made over 100,000 deposits. In addition to providing financial aid to protesters, the fund was also used to sponsor pro-democracy rallies such as paying for audio equipment.

The Vatican has remained largely muted over Zen’s case apart from a statement in May, which said it learned of Zen’s arrest with “concern” and was “following the evolution of the situation with extreme attention.”

On September 14, on a return flight from Kazakhstan, Pope Francis was asked whether he considered the looming trial against Zen a violation of religious freedom.

In a convoluted answer, the Pope repeatedly stressed his support for “the path of dialogue” and the importance to respect “the Chinese mentality.” He also refused to qualify China as undemocratic “because it’s such a complex country,” according to the Vatican News.

“Yes, it is true that there are things that seem undemocratic to us, that is true,” the Pope said. “Cardinal Zen is going to trial these days, I think. And he says what he feels, and you can see that there are limitations there.”

Zen’s trial comes at a sensitive time for the Vatican, which is preparing to renew a controversial deal with Beijing over the appointment of bishops in China. Under the original deal struck in 2018, the Vatican recognized the legitimacy of seven bishops appointed by the Chinese government. The deal came at a time when China was doubling down its crackdown on underground Christian groups as part of leader Xi Jinping’s campaign to bring religion under the absolute control of the Communist Party.

Zen has openly criticized the deal, calling it an “incredible betrayal” and accusing the Vatican of “giving the flock into the mouths of the wolves.”

In 2020, the Vatican said the deal was extended for another two years.

‘Conscience of Hong Kong’

Born to Catholic parents in Shanghai in 1932, Zen fled to Hong Kong with his family to escape looming Communist rule as a teenager. He was ordained as a priest in 1961 and made Bishop of Hong Kong in 2002, before retiring in 2009.

Known as the “conscience of Hong Kong” among his supporters, Zen has long been a prominent advocate for democracy, human rights and religious freedom. He has been on the front lines of some of the city’s most important protests, from the mass rally against national security legislation in 2003 to the “Umbrella Movement” demanding universal suffrage in 2014.

Zen’s prosecution is the latest in an ongoing crackdown on Hong Kong’s pro-democracy movement, which saw the Asian financial hub rocked by street protests through much of 2019 in resistance to Beijing’s tightening grip.

Beijing responded by imposing a controversial national security law in 2020, which critics say has been used to crush the city’s opposition movement, overhaul its electoral system, silence its outspoken media and cripple its once-vibrant civil society. Most of Hong Kong’s prominent pro-democracy figures have either been thrown in jail or gone into exile.

The Hong Kong government has repeatedly denied the national security law is suppressing freedoms. Instead, it insists the law has ended chaos and restored stability to the city.

CNN’s Delia Gallagher contributed to this report.