William Haddow Pettit, who was born on 13 April 1885 in Nelson, New Zealand, was the son of Thomas Pettit, a grocer, Baptist, temperance advocate and (later) city councillor, and his wife, Isabella Haddow. William was from youth a brilliant student and skilful debater. Deeply impressed by the American ecumenical leader John R. Mott, who visited New Zealand in 1896 and 1903, he began an ardent commitment to what was to become the Student Christian Movement, first at Nelson College and then as a medical student at Otago University from 1904 to 1908. So it was no surprise when he and his wife, Letitia Greacen Campbell, a nurse, went out as medical missionaries for the New Zealand Baptist Missionary Society to East Bengal (Bangladesh) just after their marriage in Dunedin on 14 October 1910.
For William Pettit the years in India were an exhilarating experience and an opportunity to ponder theological questions; he also assisted in Mott's evangelistic campaign in Calcutta in 1912. For Letitia the birth of two children in India was difficult and she returned to Dunedin for the birth of their third daughter in 1914. A son was born some years later.
When he returned to New Zealand in 1915, Pettit volunteered as an officer in the New Zealand Medical Corps at Featherston Military Camp, where he attained the rank of major. He wasted no time in persuading the army that he should lecture soldiers on the perils of venereal disease. They also agreed to publish a booklet he wrote on the subject. Although soldiers largely ignored his advice, his strictly moralistic lectures were soon in demand by the general public, and at the urging of Surgeon General R. S. F. Henderson the army appointed him a health lecturer. In the 1918 influenza epidemic he became a local hero in Upper Hutt at the temporary hospital he operated in the primary school. He was appointed an MBE in 1919.
In 1919 Pettit went into general medical practice in Auckland. By this time his theological position had become very fundamentalist and he appeared as a leading public advocate of biblical inerrancy, and of opposition to evolution and 'modernism'. He fought many battles with rationalists and liberals in ensuing years, and maintained his causes in public meetings and regular newspaper advertisements. He had strong support from Joseph Kemp, pastor of the Auckland Baptist Tabernacle, yet Pettit left the Baptist church and joined the Open Brethren, strongly subscribing to that denomination's separatist stance, biblical fundamentalism and distinctive form of ministry and worship.
Above all he found himself sharply at odds with his colleagues in the New Zealand Student Christian Movement, which increasingly advocated the liberalisation of theology. He nevertheless maintained a close involvement with the movement, running a conservative bookstall at its conferences and joining in fierce wrangles over the inerrancy of the Bible and the objective of missions. These disputes reached their culmination after the 1926–27 conference, when Pettit resigned and took steps to found a new fundamentalist student group in Auckland.
In 1930 he sponsored the visit of Howard Guinness on behalf of anti-modernist evangelical groups which had broken away from the Student Christian Movement in Britain. Pettit encouraged Guinness to speak in secondary schools, and this initiative led to the Crusader Movement, which successfully organised evangelical groups in most state secondary schools. Pettit was the founding chairman of the new organisation (initially called the Crusader Union of New Zealand) and invited Dr J. M. Laird, a hero of the 1931 Hawke's Bay earthquake, to be its general secretary. Groups formed more slowly in the university colleges, but in 1936 Pettit was present when they formed the Inter-Varsity Fellowship of Evangelical Unions (NZ), whose constitution forbade joint activities with any other organisation.
Pettit was always courteous and charming, yet he was always a fighter, utterly opposed to compromise. His fragile and gaunt figure was often seen visiting opposing groups, gathering information before he attacked them. As time went on he became even more conservative. He consistently opposed ecumenism and the World Council of Churches, and disapproved of American evangelist Billy Graham's 1959 New Zealand crusade being sponsored by the National Council of Churches in New Zealand (yet he served as medical consultant to Graham's team). He resigned, without rancour, from the Scripture Union's council in 1958 over these and other interdenominational issues. In the 1960s he became notorious as an opponent of the charismatic movement. By this stage Pettit had lost favour with most other evangelicals, who preferred a more sensitive approach towards their denominations. He remained an extremist to the end of his long life, advocating conscientious separation from evil and the defence of the Bible in the public arena.
Letitia Pettit died in 1965. William Pettit spent the last years of his life in an Auckland private hospital, and died there a centenarian on 16 December 1985. Characterised as 'Mr Valiant-for-Truth' (from Bunyan's Pilgrim's progress ), he had constantly defended his beliefs with zeal and intensity. Through his efforts fundamentalism played a significant role in the religious development of New Zealand.