Joshua Wong is 19 years old. A committed Christian, he last year found himself at the centre of the Umbrella Movement, when tens of thousands of people occupied the streets of Hong Kong in protest at Beijing's grip on the city's electoral system.

Next week, he could be sentenced to five years in jail.

The university student and founder of Scholarism, a pro-democracy activist group, mobilised thousands of his peers to demonstrate against the Chinese government's insistence on screening political candidates in Hong Kong to ensure their allegiance to the Communist party.

It had been hoped that open elections would be held in the city in 2017, but a motion ruling against it was passed last August. The move caused outrage; it was considered by many to contradict Beijing's promise to one day allow Hong Kong universal suffrage; a pledge it had made to the UK before the city was given back to mainland China in 1997.

What began as a student-led strike and mass sit-in evolved into a monumental movement, as a mass, peaceful campaign already organised by Occupy Central was brought forward to capitalise on momentum. Some 100,000 people turned out to protest outside government buildings and occupy a number of major intersections, overwhelming the city's police force.

Wong was arrested for his part in the campaign, which went on to last 79 days in total. He has been charged with inciting unauthorised assembly, obstructing a police officer and contempt of court, and will face trial next week along with over 100 of his fellow protest leaders.

In an interview with Christian Today, Wong dismissed the accusations levelled at him by Chinese state media, which has branded him an "extremist". He certainly doesn't look like one – he's slight, wears thick-rimmed glasses and sports a school-boy haircut. But though he might be young, he's in demand. He speaks at a frighteningly quick pace as we walk through the rain in central London; he doesn't have time to sit-down for an interview, but is determined that his message is heard.

"What we're fighting for is just for the Chinese government to uphold the promise in the 1984 declaration of 'one country two systems'; true democracy under the agreement of Britain and China," he said.

"The Chinese government has broken that promise."

Wong is in the UK for a speaking tour of six universities, and his trip coincides – by chance, he says – with Chinese President Xi Jinping's official state visit to London. Campaign groups have expressed concern that the UK is turning a blind eye to China's human rights record in order to strengthen diplomatic and economic ties, and protested Xi's lavish welcome in St James' Park yesterday.

Wong joined them, and has urged the British government not to prioritise business over human rights, both in Hong Kong and the mainland. "I want David Cameron to know that Britain should raise the importance of universal freedom and democracy," he said.

"Britain claims that it fought World War Two for freedom and democracy, but my view is that David Cameron is ignoring that. I would say to him, 'Don't let the benefit of investment blind your eyes'.

"I think Britain bears a special responsibility towards the issue in Hong Kong, and the government should not keep silent."

Wong was raised a Christian, and says his faith has strengthened his determination to fight for justice. When he was a boy, his father took him to visit poor communities in Hong Kong, and instilled in him a passion to advocate on behalf of the vulnerable.

"I believe in Christ. I believe everyone [is] born equal, and they're loved by Jesus," he told reporters during the demonstrations last year. "I think that everyone should get the equal rights in our political system. We should care for the weak and poor in our society."

Speaking to Christian Today, Wong added that he saw the Umbrella Movement as an opportunity to live up to his biblical namesake. "The Bible teaches us that we need to fight for justice, and Christians bear the responsibility to be salt and light in society," he said. "We have more obligation and a more important role in the world other than being just a normal citizen in society who wants to earn money."

Though he may be jailed for his commitment to true democracy, Wong shows no sign of slowing down – there's still a long way to go, he says.

"I am still optimistic, that's why I'm still here on the frontline. The fight for democracy in Hong Kong has been a long-term battle, and a real struggle in the face of indifference from the Communist party. What I hope to do is fight for the future."

Wong is currently considering whether to run in Hong Kong's Legislative Council elections next year, the equivalent of campaiging to be an MP, which would make him the youngest person to do so. He believes its time to make the demands of the Umbrella Movement heard within the parliament of Hong Kong itself.

"We need to get more influence and stronger power, and bring our demands from the street to the institution," he explained.

"We're still fighting for the future, and we will fight until we get what we want."