Originally a zoologist, he later turned his attentions to widening the influence of conservative evangelical Christianity within universities and theological colleges.
Then a pacifist, he remained at Cambridge to study for a Doctor of Philosophy (PhD) degree in zoology rather than fight in the Second World War.
[1] Intending on an academic career, Barclay unsuccessfully applied to become a Fellow of Trinity College, Cambridge after his completed his doctorate in 1944.
[1] In 1945, he was employed as an assistant secretary of the Inter-Varsity Fellowship of Evangelical Unions (IVF): this was intended as a two-year temporary position, but he would spend his whole career with the organisation.
[1] Among his early actions was securing Tyndale House in Cambridge, which had once belonged to the Barclay family, as an independent centre for biblical research.
[1] He remained active, however, serving as honorary vice-president of IFES from 1983 to 1991 and was co-founder of the Science and Christian Belief academic journal in 1989.
[2][5] In 1945, Barclay was admitted to the office of Reader by the Bishop of Ely, and exercised a lay preaching ministry in the Church of England.