The head of Hong Kong's Catholic Diocese on Monday said he was in disbelief at learning he will become a cardinal.

Bishop Stephen Chow was one of 21 clerics named as cardinals-to-be by Pope Francis on Sunday. A ceremony will take place in September to formalise their appointments.

Speaking to reporters at the Cathedral of the Immaculate Conception, Chow said the elevation will give him a new mission.

"I continue to hope that there will be more reconciliation and more hopes for young people, especially those who have been in jail, so they have a future. That's very important," he said.

Some of the city's Catholics told RTHK they are thrilled about the news.

"It is the honour of the Hong Kong Catholic Diocese, and a day for the Chinese Catholic church to celebrate," said a man surnamed Leung.

A woman surnamed Lau said she hopes Chow will continue to lead by example.

"I think he is fit for the role. He is kind and he cares about Catholics... I hope he can lead Catholics to love our lives and our religion," she said.

Kung Lap-yan, a theologian and associate professor at Chinese University, said while it is not the first time a Hong Kong bishop has been made a cardinal, the move implies that the city will play a larger role in Vatican affairs in future.

"The role of the Catholic church in Hong Kong would be more important in the eyes of the Vatican, in order to have a more recognised position to keep the relationship with the Catholic church in mainland China," he said.