James George Deck was born on 1 November 1807, in Bury St Edmunds, Suffolk, England, the son of Mary Welch and her husband, John Deck, who were members of the Church of England. James received a military training in Paris, and then purchased a commission in the 14th Madras Native Infantry of the East India Company, serving as lieutenant from 1824 to 1826. Returning to England, he experienced an evangelical conversion and, before his return to India in 1830, attended a private Church of England theological college. Probably on 22 April 1829, at Westbury, now part of Clifton, Somerset, James Deck married Alicia Feild, daughter of his tutor; they were to have nine children.
Conscientious scruples about military service led Deck to resign his commission in 1835, and after his return to England he was rebaptised, contrary to Church of England teachings. James and Alicia Deck were drawn into the radical, anti-denominational Plymouth Brethren, and Deck became an evangelist in the vicinity of Taunton, later moving to Weymouth. A sensitive and semi-mystical thinker, he wrote religious poetry which was used in the movement's earliest hymn books. As divisions emerged in the Brethren between a more sectarian Exclusive party and a more ecumenical Open party, Deck sought to reconcile the opponents, but his two tracts on the subject won him no friends. He then suffered a stroke and partial paralysis which led him to emigrate to New Zealand.
Deck, his wife and eight surviving children arrived on the Cornwall in Wellington in 1853. They purchased land in the Waiwhero district, eight miles from Motueka, in Nelson province. Alicia Deck died of an ear abscess in December 1853, and on 17 July 1855 James Deck married Lewanna Atkinson at Motueka. He became well known as a preacher, advocate of Christian unity and debater with other nonconformists. A cluster of Brethren assemblies developed between Nelson and Motueka.
In May 1865 Lewanna Deck and her fifth and youngest child died of measles, and Deck moved to Wellington, where his preaching and advocacy of church reform made a large impact among lay people, although it outraged their ministers. He made evangelistic tours of several regions, preaching in churches of many denominations. As a result the Brethren movement had a far more significant impact on church life in New Zealand than it had in England. Informal assemblies were founded in many towns. Deck, with his long white beard, balding head, mystical temperament and delicate health, exercised an informal and unassertive leadership among them.
Deck deliberately neglected to maintain contacts with any English Brethren, and never explained to his churches the issues which had split the English church. His dereliction became known in England when his ecumenically minded son John Feild Deck returned there to study medicine. His father's Exclusive Brethren friends were angered to hear that the New Zealand assemblies practised an open attitude towards other Christians. Under pressure, Deck wrote to Exclusive Brethren leaders in England in 1872, confessing his errors. Many of his churches, particularly those in the south led by his sons Samuel and John, were not so willing to submit to sectarian regulations.
Visits to New Zealand by leading English Exclusive Brethren, George Vicesimus Wigram in 1874–75 and 1877, and John Nelson Darby in 1875–76, restored Deck to favour, but the churches associated with his sons separated over the issue. Deck nevertheless maintained warm relations with his older sons and their churches. Until his death on 14 August 1884 he lived at Sandridge, in Thorp Street, Motueka, where his daughters ran a school. The Deck family remained prominent among Exclusive Brethren, but was also respected in the broader evangelical world, for John and his wife, Emily Young, moved to Australia in 1877 and helped in the formation of the Queensland Kanaka mission (later the South Sea Evangelical Mission), the first Brethren missionary body to be founded in the antipodes.
Deck's sensitive temperament, his warm disposition, and his fervent preaching made the Plymouth Brethren a significant if sectarian force in New Zealand. James Deck was one of the most original of the religious pioneers in New Zealand. Because he owed allegiance to no foreign church or theology, his assemblies adapted to the local environment to a degree which was rare among colonial churches. He was an intelligent thinker, aware of the problems of religious reform movements. His gifts as a poet and hymn writer, evident in his Hymns and sacred poems, were of a high order. His style was simple; exalted religious sentiments were expressed in unobtrusive rhythms and simple language. The poems are, however, little known outside the context of Brethren worship, where they remain universally popular.