Killen, William Dool (1806–1902), presbyterian church historian, was born 5 April 1806 in Ballymena, Co. Antrim, elder son of John Killen, merchant, and Martha Killen (née Dool). Through his paternal grandmother, Blanche Brice, he could claim descent from Edward Brice (qv) of Ballycarry, the first presbyterian minister installed in an Ulster parish in the seventeenth century.
Educated locally in a school connected with the presbyterian congregation in Ballymena, in which his father was an elder, and later in the grammar school which became Ballymena Academy, he entered the Belfast Institution in 1821. He took the Institution's general certificate in arts in 1824 and spent a year in Co. Cavan as tutor to two brothers preparing to enter TCD. He was deeply impressed by the piety of the primitive methodist family in which he lived as tutor, and began ‘to contemplate religion with new eyes’. Returning to the Belfast Institution, he completed his studies for the presbyterian ministry and was licensed by the Ballymena presbytery in October 1827. Rejected by the Co. Antrim congregation of Kilraughts, who suspected his theological orthodoxy, he was ordained and installed in Raphoe, Co. Donegal, on 11 November 1829. The following year he married Anne (d. 1886), third daughter of Thomas Young of Ballymena.
Through the good offices of Bishop William Bissett of Raphoe he was able to use the diocesan library to pursue his continuing studies in church history. He became involved in ecclesiastical controversy provoked by a Church of Ireland curate's attack on presbyterianism, and contributed to the vigorous presbyterian polemics of Presbyterianism defended and the Plea of presbytery. Having thus demonstrated his historical scholarship he was appointed to the chair of church history vacated by James Seaton Reid (qv) in 1841.
It was also in 1841 that the newly formed general assembly decided that the time had come when their church should have its own theological college, and this was achieved when a presbyterian theological faculty was constituted in 1847 and the Presbyterian College was opened in 1853. Killen taught church history in the college for almost fifty years and in 1869 succeeded Henry Cooke (qv) as the college's president.
Killen was a theological conservative who sympathised little with nineteenth-century developments in biblical criticism and science. As a lecturer he was uninspiring, particularly in his later years, but he made a significant contribution to ecclesiastical historiography through his many and substantial publications. Chief among them were his completion of the third volume of Reid's magisterial History of the Presbyterian church in Ireland (1853, 1867), his memoir (1867) of John Edgar (qv), father of the Irish presbyterian temperance movement, and his Ecclesiastical history of Ireland (2 vols, 1875). Of particular interest is his autobiography, Reminiscences of a long life (1901), covering the main events and personalities in nineteenth-century Irish presbyterian history. He also edited, with introductions and explanatory notes, such valuable seventeenth-century writings as John Mackenzie's (qv) Siege of Derry (1861) and Patrick Adair's (qv) Rise and progress of the Presbyterianism Church in Ireland (1866).
The contributor to Presbyterianism defended remained an active presbyterian polemicist throughout his life, controverting episcopalian apologists from Ignatius in the second century to J. B. Lightfoot in the nineteenth. No lover of Roman catholicism, he nevertheless helped to persuade the general assembly that catholics who became presbyterians should not be rebaptised, as they were already members of the church visible. His hostility to unitarianism is evident in his treatment of the Arian controversy in his completion of Reid's History, and he was in error when he claimed that it was the Arians and non-subscribers who were United Irish rebels in 1798.
He received honorary doctorates from Glasgow University – DD (1843) and LLD (1901) – but was never elected moderator of the general assembly. He died of pneumonia 10 January 1902, survived by four of his eight children (three sons and a daughter), and is buried in Balmoral cemetery in Belfast. He bequeathed his large library to the college he had served for so many years.