Labour leader Ed Miliband was among guests at the Christian Socialist Movement's 50th birthday party at Church.co.uk in Kennington Road on Monday night.
"People who make change happen are not politicians," he told guests in an appeal for the participation of churchgoers in politics.
"We have to practise ethics in the organisation of our party," he added.
Messages were read out from Ed Miliband's two predecessors Tony Blair and Gordon Brown. Alun Michael MP introduced the Labour Leader as "the next prime minister".
Ed Miliband, who is due to marry his partner Justine on Friday, was presented with a framed drawing of RH Tawney as a wedding present. The Christian socialist academic lived in Bloomsbury where a series of meetings at the Lamb pub led to the formation of CSM in 1961.
Earlier in the day the Labour Leader was at the Royal Festival Hall to speak about 'the Promise of Britain' in a new bid to confront the government. He suggested that what unites everyone he meets around the country was that there seemed to be so much that politics does not talk about.
"That's why I say people want more from us," he said. "People want more from our politics.
"And what is happening to the next generation is one of those unspoken truths that people know about – but somehow politicians seem to refuse to discuss."
It was not all his opponents' fault he conceded.
"Some of these are big problems that are rooted in the way the country's been changing for years, not just over the last year.
"But my criticism of this government, of David Cameron and Nick Clegg, is that they are doing nothing to turn things around."
Looking to a Labour victory he said: "As prime minister I will judge the next Labour government on the opportunities that Britain can provide for all of the next generation."
As well as his two engagements in SE1 on Monday, the Labour leader also visited the new Michael Faraday Primary School on the Aylesbury Estate in Walworth.