Answer by taking a quote from the following article:

Cronin was born in Cardross, Dunbartonshire, Scotland, the only child of a Protestant mother, Jessie Cronin (nee Montgomerie), and a Catholic father, Patrick Cronin. Cronin often wrote of young men from similarly mixed backgrounds. His paternal grandparents had emigrated from County Armagh, Ireland, and become glass and china merchants in Alexandria. Owen Cronin, his grandfather, had had his surname changed from Cronague in 1870.

The Citadel (1937), a tale of a mining company doctor's struggle to balance scientific integrity with social obligations, helped to incite the establishment of the National Health Service (NHS) in the United Kingdom by exposing the inequity and incompetence of medical practice at the time. In the novel Cronin advocated a free public health service in order to defeat the wiles of those doctors who "raised guinea-snatching and the bamboozling of patients to an art form." Dr Cronin and Aneurin Bevan had both worked at the Tredegar Cottage Hospital in Wales, which served as one of the bases for the NHS. The author quickly made enemies in the medical profession, and there was a concerted effort by one group of specialists to get The Citadel banned. Cronin's novel, which was the highest-selling book ever published by Victor Gollancz, informed the public about corruption within the medical system, planting a seed that eventually led to reform. Not only were the author's pioneering ideas instrumental in the creation of the NHS, but the historian Raphael Samuel asserted in 1995 that the popularity of Cronin's novels played a substantial role in the Labour Party's landslide victory in 1945.  By contrast, according to one of Cronin's biographers, Alan Davies, the book's reception was mixed. A few of the more vociferous medical practitioners of the day took exception to one of its many messages: that a few well-heeled doctors in fashionable practices were ripping off their equally well-off patients. Some pointed to the lack of balance between criticism and praise for hard-working doctors. The majority accepted it for what it was, a topical novel. The press attempted to incite passions within the profession in an attempt to sell copy, while Victor Gollancz followed suit in an attempt to promote the book, all overlooking the fact that it was a work of fiction, not a scientific piece of research, and not autobiographical.  In the United States The Citadel won the National Book Award, Favorite Fiction of 1937, voted by members of the American Booksellers Association. According to a Gallup poll conducted in 1939, The Citadel was voted the most interesting book readers had ever read.

was he involved in politics