Brought up as an engineering smith, his love of reading led him to leave this calling and set up a school in his native village.
In 1861 Spears attracted the attention of Robert Brook Aspland, was invited to London by Sir James Clarke Lawrence, and became (1862) minister of Stamford Street chapel, Blackfriars.
In 1867 he was elected co-secretary of the British and Foreign Unitarian Association with Aspland, on whose death (1869) he became general secretary, reviving it and nearly quadrupling its income.
He was interested in the monotheistic movement of the Brahmo Somaj of India, and was in contact with its leaders from the visit (1870) to Britain of Keshub Chunder Sen, who was his guest.
On his initiative was founded (7 June 1881) the Christian Conference, brought together representatives of all denominations, from Cardinal Manning to James Martineau.
He wrote "The following are the leading articles of the religion of Christian Unitarians: They believe — Of the Holy Scriptures: "Thy word is a lamp unto my feet, and a light unto my path."
1873); compiled from Joseph Priestley's works The Apostolic and Primitive Church … Unitarian (1871); and wrote the introduction and appendix to Hugh Hutton Stannus's History of the Origin of the Doctrine of the Trinity (1882).