Arianism is the theological teaching attributed to Arius (ca. AD 250–336), a Christian presbyter in Alexandria, Egypt, concerning the relationship of the persons of the Trinity ('God the Father', 'God the Son', and 'God the Holy Spirit') and the precise nature of the Son of God as being a subordinate entity to God the Father. Deemed a heretic by the First Council of Nicaea of 325, Arius was later exonerated in 335 at the First Synod of Tyre, and then, after his death, pronounced a heretic again at the First Council of Constantinople of 381. The Roman Emperors Constantius II (337–361) and Valens (364–378) were Arians or Semi-Arians.

The Arian concept of Christ is that the Son of God did not always exist, but was created by—and is therefore distinct from—God the Father. This belief is grounded in the Gospel of John passage “You heard me say, ‘I am going away and I am coming back to you.’ If you loved me, you would be glad that I am going to the Father, for the Father is greater than I." (verse 14:28)

Diarmaid Ninian John MacCulloch,Knight,FBA, FSA, FRHistS (born 31 October 1951) is a professor of history of the church at the University of Oxford (since 1997) and Fellow (formerly Senior Tutor) of St Cross College, Oxford (since 1995). Though ordained as a deacon in the Church of England, he declined ordination to the priesthood because of the church's attitude to his homosexuality. In 2009 he encapsulated the evolution of his religious beliefs: "I was brought up in the presence of the Bible, and I remember with affection what it was like to hold a dogmatic position on the statements of Christian belief. I would now describe myself as a candid friend of Christianity."

Diarmaid MacCulloch was born in Kent, England. He attended Hillcroft Preparatory School and Stowmarket Grammar school in Suffolk. He subsequently studied history at Churchill College, Cambridge, where he subsequently obtained a BA degree in 1972 and an MA degree 1976. During that period, he was also organ scholar at the college. He then completed a Diploma in Archive Administration at Liverpool University in 1973 and then returned to Cambridge to complete in 1977 a PhD degree in Tudor history under the supervision of Sir Geoffrey Rudolph Elton, combining this with a position as Junior Research Fellow at Churchill College. MacCulloch was elected a Fellow of the Society of Antiquaries of London (1978), a Fellow of the Royal Historical Society (1982) and a Fellow of the British Academy (2001). He furthermore received a DD degree of the University of Oxford (2001) and in 2003 was awarded an honorary DLitt degree by the University of East Anglia. He co-edits the Journal of Ecclesiastical History.