A 90-year-old Roman Catholic cardinal, who was arrested earlier this year by the Chinese Communist Party (CCP) for allegedly colluding with foreign forces by failing to register his pro-democracy charity fund with the government, begins his formal trial Monday in Hong Kong.

Cardinal Joseph Zen, the former archbishop of Hong Kong, was unceremoniously arrested with four other pro-democracy advocates in May for allegedly colluding with foreign forces in violation of a CCP-imposed national security law, according to Crux.

The five were specifically charged with not registering a now-defunct relief fund used to bail out protesters and pay the legal fees of Hong Kongers who demonstrated in 2019 against a controversial bill allowing extradition to mainland China.

In addition to Zen, others on trial include Barrister Margaret Ng, singer-activist Denise Ho, scholar Hui Po-keung, former legislator Cyd Ho and activist Sze Ching-wee, who served as the fund’s secretary general before it shut down last October. All five submitted pleas of not guilty. Zen has been out on bail since May 11.

ARRESTED CARDINAL REPEATEDLY WARNED HOLY SEE OF CHINESE PERSECUTION, CONDEMNED VATICAN'S SECRETARY OF STATE

Retired Cardinal Joseph Zen

Joseph Zen, cardinal of the Roman Catholic Church, arrives for a church service at St. Andrew's Parish in Hong Kong on Friday, June 4, 2021. (Chan Long Hei/Bloomberg via Getty Images)

"I want to stress that Hong Kong is a society with rule of law where no organization or individual is above the law and all offenses shall be prosecuted in accordance with law," Chinese Foreign Ministry spokesperson Zhao Lijian said at the time. "We firmly oppose any act that denigrates rule of law in Hong Kong and interferes in its affairs."

Frank Wolf, a former U.S. congressman from Virginia who serves on the U.S. Commission on International Religious Freedom (USCIRF), recently penned an op-ed for Fox News Digital in which he called the charges against Zen "spurious," and wrote that he is "deeply concerned about his safety and well-being."

CARDINAL ZEN APPEARS IN HONG KONG COURT AFTER ARREST BY CHINESE COMMUNIST PARTY

"The Chinese government has broken its promises and is eviscerating the rule of law and civil liberties in Hong Kong," Wolf wrote. "Religious freedom as a fundamental right will inevitably suffer as well. In fact, we are witnessing before our eyes this free city devolving into an increasingly repressive society where no one resisting government tyranny is safe, including religious leaders and communities."

"Cardinal Zen is not only a democracy activist in Hong Kong, but he has also been a tireless and outspoken advocate for religious freedom and human rights in China," Wolf also said. "He is a hero and role model for many."

Hong Kong Catholic Bishops

(L to R) Cardinals John Tong Hon, Cardinal Joseph Zen, Bishop Stephen Chow, Bishop Joseph Ha and Dom. Paul Gao pose for a photo at the Episcopal Ordination of the Most Reverend Stephen Chow in Hong Kong's Cathedral of the Immaculate Conception on Dec. 4, 2021. (BERTHA WANG/AFP via Getty Images)

"USCIRF is paying close attention to the trial against Cardinal Zen and his associates," said USCIRF Chair Nury Turkel. "We remain concerned that Hong Kong and Chinese authorities will use the National Security Law to arbitrarily prosecute not only Cardinal Zen, but many other religious individuals and organizations who were motivated by their faith to participate in democracy protests in Hong Kong."

Zen's repeated warnings to the Vatican regarding the CCP's increased crackdowns on religious minorities and public figures who resist the government have historically been met with little response, prompting the cardinal to blame Vatican Secretary of State Pietro Parolin for keeping church leaders in the dark on such issues.

Zen has also criticized Pope Francis, whose South American background he has blamed for the Holy See's relaxed diplomacy with China, saying that the pope is not familiar with "communists in power."

ARRESTED CARDINAL REPEATEDLY WARNED HOLY SEE OF CHINESE PERSECUTION, CONDEMNED VATICAN'S SECRETARY OF STATE

"I try to explain, remembering that Pope Francis comes from South America, where the communists are the good guys defending the poor from the oppression of military regimes in collusion with the rich, so he may have sympathy for them," Zen said in 2020. "He doesn’t have direct experience of communists in power, oppressors of peoples."

Pope Francis in Kazakhstan

Pope Francis meets with bishops, priests, deacons, consecrated persons, seminarians and pastoral workers in Our Lady Of Perpetual Help Cathedral in Nur-Sultan, Kazakhstan, on Sept. 15, 2022. (REUTERS/Pavel Mikheyev)

Chinese President Xi Jinping reportedly turned down an offer to meet with Pope Francis while the two were in Kazakhstan at the same time last week, according to Reuters.

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Fox News' Timothy H.J. Nerrozzi contributed to this report.