Historian Martin Wellings states: His combination of modernist theology, high sacramentalism, and Socialist politics, expressed with insouciant wit and unapologetic élan, thrilled audiences, delighted admirers, and reduced opponents to apoplectic fury.

[citation needed] Soper offered as a candidate for the Methodist ministry, and while still a probationary minister (in his first appointment), he sought larger congregations by taking to open air preaching in imitation of the founders of Methodism.

From 1926 until well into his nineties, he preached at London's centres for free speech, Tower Hill and (from 1942) Speakers' Corner in Hyde Park; he was often referred to as "Dr Soapbox" in honour of the outdoor preacher's chief piece of apparatus.

He joined the Peace Pledge Union in 1937 and preached pacifism throughout the Second World War, being deemed so effective that he was banned from broadcasting on the BBC.

[5] He became the first Methodist minister to sit in the House of Lords, an institution whose existence he opposed (he referred to it as "proof of the reality of life after death")[6] but which he was able to use as a platform for the expression of his views.

In 1978 he spoke in depth about his time as a controversial figure on a soapbox at Hyde Park Corner and his hopes for the future in the BBC radio programme Quest into 1978 with priest and journalist[8] Owen Spencer-Thomas.