CMS missionary Max Collison will be telling Sydney teenagers how a simple cup of coffee can be part of the light that leads Kenyans out of slums and into new life.

Mr Collison is speaking at the CMS youth camp, CMSpresso, in Katoomba next month, from May 19 to 21.

One of his key points will be encouraging youth to apply a Christian sense of justice to the city's café culture.

"Coffee is a black and white issue," says Mr Collison, enjoying the pun.

"When you walk into a major supermarket in Australia you can know for sure that no-one was fairly paid for the growing and picking of the coffee on sale there."

Mr Collison is teaching CMS audiences that a key part to showing the sort of "light' the Apostle Paul describes to the Ephesians is embracing justice as a community.

"We together are light, and the key thing about light is it's bleedingly obvious," he recently told a CMS Summer School audience in Canberra.

"We should be so obviously different to the rest of the world that they see us and recognise their own darkness."

He has been preaching against supporting a commercial system that sees coffee growers in Kenya paid around 16 cents a kilogram for their crop " and coffee pickers, even less.

Mr Collison says the connection between what sort of coffee Christian's drink and the message they preach should be obvious.

"It is by what we do that we show what we say is true," he says.

As a CMS missionary, Mr Collison was instrumental in setting up a card manufacturing business in Nairobi that lifted impoverished women out of prostitution and into economic independence.

"They wondered why we wanted to do this. All of those women are now Christians," he told his Canberra audience.

Mr Collison believes purchasing fairly traded coffee and pressuring retailers to provide it on their shelves will lead to a similar witness in Sydney.

"When people ask why, you can say "Because we love the pickers’," he says.

A wide range of products stamped with the "Fairly traded' trademark are already available in supermarkets in the United Kingdom, but Australian chains have been slow on the up-take.

"The church is the largest grass roots organisation in the world," Mr Collison says.

"We can change the economics of shopkeepers. We are enough of a market to do that."

"I long for us to be a light as we were in the evangelical awakening so that people will see that light and be attracted to the body of Christ."

Click here to see a video of the message Mr Collison will be delivering to teenagers in Katoomba next month.