Stand by for an unholy row as the Pope comes out as perhaps the world's most effective environmental campaigner.
On Thursday, Pope Francis will issue the first-ever encyclical on the environment. The most authoritative document a Pontiff can publish, this encyclical is also the first to try to influence a global political process. It is unapologetically aimed at helping to forge global agreement to tackle climate change at a special summit in Paris in December.
Circulated to the church's 5000 bishops and 400,000 priests, it is a call to action for the world's 1.2 billion Catholics. Prayer vigils and pilgrimages, protests and policy briefings have all been prepared, as environmentalists and the faithful alike seek to maximise its leverage.
The Pope himself will front the campaign, travelling to the US in September to speak at the United Nations and metaphorically entering the lions' den by addressing Congress - about a third of whose members are Catholics, but which remains the world's most resistant legislative body to taking action on global warming.