The Filipino environmentalist Yeb Saño is just one of many of the world’s 1.2bn Catholics who, on 18 June, will read Pope Francis’s encyclical on the environment which is likely to call for strong action on climate change.

Saño is in Sydney as a spiritual ambassador for OurVoices, a faith-based climate activist group that aims to “[bring] faith to the climate talks”. He predicts the pope’s letter will be “strong on stewardship, on economic justice, and the moral responsibility for all of us to be a part of caring for creation”.

In 2012, Saño shot to worldwide fame when a video of his emotional plea to the UN climate talks went viral. At the time he was leader of the Philippines’ delegation, with the country still reeling from the devastating impact of typhoon Haiyan. Saño broke down in tears halfway through his address.

He will walk with other members of Sydney’s faith communities across the harbour bridge on Friday, as part of the people’s pilgrimage, a worldwide, multi-faith climate campaign. He says religion helps bring a weighty moral dimension to “the biggest problem we face as a human family”, one that has “been missing” for more than two decades.

Yeb Sano at the UN climate conference in Warsaw in 2013. Photograph: Alik Keplicz/AP

Jacqui Remond, director of Catholic Earthcare Australia, the ecological agency for the Catholic church in Australia, will walk beside Saño. She says the Catholic faith sees creation as “a gift from God” and inspires church leaders to speak out strongly about climate change.

The pope’s last encyclical, released in 2013, was 88 pages long. What the letters lack in frequency, they more than make up for in importance, written when the pope feels he has something vital to say about church doctrine.

The topic of this encyclical was determined in 2014, after the pope’s March visit to Tacloban, a Philippine city devastated by Haiyan. A week before its release, the pope has already faced a backlash for his decision, with conservative US thinktanks and evangelical Christian leaders criticising him for getting involved in what they view as a “political issue”.

A spokesman for the conservative Cornwall alliance for the stewardship of creation, Calvin Beisner, said the pope should “back off” in a December interview with the Guardian.

“The Catholic church is correct on the ethical principles but has been misled on the science. It follows that the policies the Vatican is promoting are incorrect. Our position reflects the views of millions of evangelical Christians in the US,” he said.

Closer to the pope’s inner circle is Cardinal George Pell, former archbishop of Sydney and a climate change sceptic. In 2011 he addressed a powerful group of climate sceptics and argued the jury was still out on man-made climate change.

But for many Christians, such as Remond, science and religion are not incompatible world views. “George Pell holds an opinion that is not congruent with the science,” she says. “That’s his opinion and he’s entitled to it. But it’s not what the majority of bishops [think].”

And if the pope’s upcoming edict feels progressive, it still follows a bishops’ position paper released by the Catholic church in Australia 10 years ago, titled Climate Change: Our Responsibility to Sustain God’s Earth.

Remond says strong faith is built on intellectual, emotional and psychological rigour, and that to “seek God in our world, lives and in all things – that is a journey of great discovery, great risk, great struggle, and times of great doubt are part of that journey.

“To come through with strong faith means that it has been seriously probed and robustly questioned.”

Faith communities bring a different dimension to the climate change discussion, she says. They “speak from different wisdom sources and unify this message of caring for the Earth. And that’s something for all of us [to] recognise and hold in whatever way.”

When the pope calls to defend “the poor of the planet”, this extends beyond humankind to include all species and resources of Earth, Remond says.

“The core message will remind us of our responsibility to respect natural balances, cycles and rhythms. We live an embodied sense of doing God’s work on Earth, aligning ourselves with that deep, spiritual way of being.”