It was a big day for Japan's Catholics — Pope Francis arrived in Nagasaki, the second Pope to ever visit Japan, after John Paul II in 1981.

Nagasaki is famous for the 1945 atomic bombing during World War II, along with Hiroshima.

It might surprise many to learn that Nagasaki has always been home to Japan's largest Catholic, or Hidden Christian community — all the way back to the late 1500s.

Seventy per cent of the Catholic community are said to have died immediately in the bombing in the Urakami valley region, north of Nagasaki city itself, which was protected by mountains from the brunt of the explosion.

I can personally attest to the strength and charm of the survivors, who I interviewed for my book, Dangerous Memory in Nagasaki: Prayers, Protests and Catholic Survivor Narratives.

By talking to members of the next generation, I have traced the ongoing development of attitudes to the trauma of the bomb.

The survivors' descendants can take comfort in the Pope's visit because he is taking on their cause against nuclear weapons.

Confronting the 'bomb as providence'

Catholics in Japan tended to perceive the bombing as one aspect of God's divine will, which prevented them from condemning the American attack.

It was difficult to speak out while the survivors were trying to put their lives back together and deal with the huge number of deaths.

The mushroom cloud over Nagasaki, taken 15 minutes after the plutonium bomb exploded on August 9, 1945. ( Hiromichi Matsuda/Nagasaki Atomic Bomb Museum )

Their reticence to speak was compounded by the suggestion of Nagasaki city residents that the atomic bomb was a punishment of the Japanese gods on the "heathen" Catholic region.

When Pope John Paul II came to Japan in 1981 he said, "War is the work of humanity, war is destruction of human life, war is death."

This message led to a transformation — Catholic survivors started to speak out about the bombing.

Kataoka Chizuko, a Catholic nun, told me that Pope John Paul II's encouragement let her realise that, "if the Pope says so, even if it is painful, our experiences should be added to the discussion."

I was therefore aware as I interviewed survivors that they had hardly spoken about their ordeal and that the trauma they had experienced after 70 years was still close to the surface.

Nakamura Kazutoshi was 11 years old at the time of the bombing and considered suicide due to his depression.

Nakamura Kazutoshi is speaking out after experiencing sickness connected to the atomic bomb. ( Supplied: Gwyn McClelland )

He said it was actually his heart sickness and liver problems stemming from the bomb which drove him to speak finally at the age of 71.

His belief in Pope John Paul II's message led him to the conclusion that the atomic bomb "was foolish humanity killing each other".

From manga to oral historian

I became interested in the atomic bombings when I received the gift of a Japanese manga-cartoon about Hiroshima as a 12-year-old, from Japanese family friends.

When I visited Nagasaki for the first time, I saw a replica ruined cathedral wall in the bomb museum and was intrigued — how had the Catholics come to be here?

St Mary's Cathedral in Nagasaki was completely destroyed by the atomic bomb. ( Getty: Galerie Bilderwelt )

I learnt Japanese and became an oral historian, spending eight years interviewing Catholic bomb survivors until 2016.

My interviews showed the survivors of the atomic bomb link their survival strongly to their community's resilience through 250 years of persecution and martyrdom.

Christians make up 1.6 per cent of Japan's population.

Matsuo Sachiko is a bright and down-to-earth woman who I met in the cafe of the Atomic Bomb Museum in 2014.

She says despite being a victim of the atomic bomb, God allowed her to live and "that was a good thing, I believe".

Her grandmother survived the bombing with her as a double survivor, having returned from an extraordinary exile in the Meiji period as a six-year-old, during what is known as the "Fourth Persecution" of the Catholics in Japan in the 19th century.

Another survivor, Ozaki Tomei, was orphaned by the atomic bomb at the age of 17 and consequently became a brother in a Franciscan order.

Ozaki Tomei's life fell apart when Nagasaki was bombed. ( Supplied: Gwyn McClelland )

I was introduced to him by another author and when I called, he suggested we meet at his hairdresser.

Conducting an interview in a hair studio was certainly an unexpected element of my work as an oral historian.

I sat behind him looking into the mirror to see him as the hairdresser worked and he told me how he left home on the morning of the bombing and said, "See-ya mum!"

Protected in a tunnel factory producing torpedoes for Mitsubishi, Ozaki emerged into a world that had been changed forever.

For these two survivors, the Pope's visit will be a chance to show how well the community understands itself today — and to remember how far they have come since 1945.

Pope Francis is visiting Japan this month. He is only the second pope to ever visit the country. ( AP: Domenico Stinellis )

They are moved by Pope Francis's decision to visit Nagasaki first and Hiroshima second, reversing the order of Pope John Paul II's visit in 1981.

The 'Rome of Japan'

Nagasaki has a special significance for Pope Francis, the first Jesuit pope.

The city was once known as the "Rome of Japan" because in 1580, the local Lord Omura Sumitada donated the port to Jesuit missionaries from Portugal.

This port began as a largely Christian enterprise with the establishment of multiple churches and the presence of many missionaries.

This was the case until Christian persecution began in earnest under the reign of Shogun Tokugawa Ieyasu in late 1613 — and the oppression didn't end until 1873.

In today's Nagasaki religious inter-marriage and dialogue is much more possible than in the past.

However, discrimination against bomb victims and on religious lines is still occasionally a problem.

Catholics hold a peace march to mark the 70th anniversary of the atomic bombing of Nagasaki. ( Getty: Jiji Press )

As the Pope visits Japan, I have no doubt the local Catholics will be effusive, especially considering the importance of the previous visit by Pope John Paul II, 38 years ago.

Pope Francis will use this visit to raise the prospect of banning nuclear weapons.

He was recently quoted by Nagasaki TV comparing the "faith of the martyrs" to "one more martyrdom, the ... atomic bomb".

It's an idea that has swung further into favour for the Vatican.

Historically, the Jesuits were known for their meetings with the powerful, including 16th century military strategist Hideyoshi Toyotomi.

I hope this Pope — who will meet the new Reiwa Emperor — will also have time to listen to at least some of the Nagasaki Catholics, themselves truly a survivor community.

Dr Gwyn McClelland has a PhD in history and Japanese studies and is a lecturer at Monash University.