After assisting in ridding the Society of Friends (Friends, Society of) (Quakers) of slavery, Hicks worked for general abolition. He urged a boycott of the products of slave labour, advocated establishment of an area in the Southwest as a home for freed slaves, and helped secure legislation that brought an end to slavery in New York state. In 1811 the first of several editions of his Observations on the Slavery of the Africans and Their Descendants was published.

One of the first to preach progressive revelation, which allowed for continuing revision and renewal of doctrinal beliefs, Hicks in 1817 successfully opposed the adoption of a set creed by the Society of Friends at the Baltimore, Maryland, yearly meeting. He was subsequently called a heretic for his opposition to Evangelicalism, which stressed established beliefs, and he was held responsible by some for the Quaker schism of 1827–28. After this separation Hicks's followers called themselves the Liberal branch of the Society of Friends, but orthodox Quakers labeled them Hicksites. The Hicksites remained isolated from other Quakers until the 20th century, when mutual cooperation began to prevail.