We need to be careful not to paint with too broad a brush:
Despite its brutality and inhumanity, the slave system aroused little protest until the 18th century, when rationalist thinkers of the Enlightenment began to criticize it for its violation of the rights of man, and Quaker and other evangelical religious groups condemned it for its un-Christian qualities. By the late 18th century, moral disapproval of slavery was widespread, and antislavery reformers won a number of deceptively easy victories during this period. In Britain, Granville Sharp secured a legal decision in 1772 that West Indian planters could not hold slaves in Britain, since slavery was contrary to English law. In the United States, all of the states north of Maryland abolished slavery between 1777 and 1804. But antislavery sentiments had little effect on the centres of slavery themselves: the great plantations of the Deep South, the West Indies, and South America. Turning their attention to these areas, British and American abolitionists began working in the late 18th century to prohibit the importation of African slaves into the British colonies and the United States. Under the leadership of William Wilberforce and Thomas Clarkson, these forces succeeded in getting the slave trade to the British colonies abolished in 1807. The United States prohibited the importation of slaves that same year, though widespread smuggling continued until about 1862.
Antislavery forces then concentrated on winning the emancipation of those populations already in slavery. They were triumphant when slavery was abolished in the British West Indies by 1838 and in French possessions 10 years later.
The situation in the United States was more complex because slavery was a domestic rather than a colonial phenomenon, being the social and economic base of the plantations of 11 Southern states. Moreover, slavery had gained new vitality when an extremely profitable cotton-based agriculture developed in the South in the early 19th century. Reacting to abolitionist attacks that branded its “peculiar institution” as brutal and immoral, the South had intensified its system of slave control, particularly after the Nat Turner revolt of 1831. By that time, American abolitionists realized the failure of gradualism and persuasion, and they subsequently turned to a more militant policy, demanding immediate abolition by law.
Probably the best-known abolitionist was the aggressive agitator William Lloyd Garrison, founder of the American Anti-Slavery Society (1833–70). Others, drawn from the ranks of the clergy, included Theodore Dwight Weld and Theodore Parker; from the world of letters, John Greenleaf Whittier, James Russell Lowell, and Lydia Maria Child; and, from the free-black community, such articulate former slaves as Frederick Douglass and William Wells Brown.https://www.britannica.com/topic/abolitionism-European-and-American-social-movement
John Wesley is well known for his opposition to slavery. In 1773 he printed a pamphlet titled ‘Thoughts Upon Slavery’, in which he decried the evils of slavery and called for slave traders and owners to repent and free their slaves. Just as Wesley preached an individualistic experiential salvation in Christ, it is often assumed that he sought to cure social ills solely in the transformation of individual souls, rather than in the transformation of society at large. For example, Forell states, “Wesley’s social teachings are a peculiar combination of political conservatism and social activism. He shares…a tendency to seek individualistic solutions to social evils and to deal with the symptoms of prevailing evils rather than their causes.” (Forell, p 181) This conclusion is an exaggeration of the facts. As we will see from ‘Thoughts Upon Slavery’ [hereafter TUS], Wesley fervently desired an end to the societal evil of slavery, and pursued this end with what he believed to be the most practical means at his disposal.https://daveduncombe.wordpress.com/2014/09/16/john-wesley-on-slavery/