During the revolutionary decade of the 1640s Thomas Collier emerged from his native Somerset to become a significant Particular Baptist leader. He produced more than a score of books and established numerous churches.

Collier was a well-known controversialist who debated opponents on subjects such as baptism and the ordination of lay preachers. Collier's theology was worked out in the heat of such debates and must be studied against that landscape to be properly understood.

Collier's writings and career reveal surprising willingness to embrace heterodox theological positions by Particular Baptist standards, especially in the late 1640s and after 1660. In the early period of his career he was enaroured of an allegorical, spiritualizing method of biblical interpretation and after 1660 he became increasingly hostile to limited atonement and election.

The most orthodox phase of Collier's career was the period between 1653 and 1659 when he served as the leader of the Particular Baptists' Western Association. Under his leadership the association produced their Somerset Confession in 1656.

After the Restoration Collier's disputes were increasingly with his fellow Particular Baptists. The publication of his Body of Divinity in 1674 and his Additional Word as a supplement to it in 1676 revealed increasingly divergent soteriological and eschatological views from those being espoused by the Particular Baptists. An attempt was made to discipline Collier by the London Baptist leadership, which was strongly and successfully resisted by Collier and his supporters within his local church in Southwick, Wiltshire.

Collier's 1678 Confession of Faith, written in response to the London Baptists' adaptation of the Westminster Confession published the previous year, illustrated the wide breach of doctrine that had developed between Collier and his denominational colleagues.