John Wesley was an early leader in the Methodist movement. Under his direction, Methodists became leaders in many areas of social justice, including prison reform and the abolition of the Slave Trade.
In 1736-7, Wesley visited North America including Georgia, which was then a British colony, and there he came into contact with enslaved people. This experience left him with a loathing of slavery but at first he felt unable to act on this. From 1739 onwards, Wesley and the Methodists were persecuted by clergymen and magistrates. They were attacked in sermons and in print and at times attacked by mobs.
The focus that Wesley needed came when Granville Sharp contested the case of a runaway slave (James Somerset) in the courts. Wesley was moved to study a text by the Philadelphia Quaker, Anthony Benezet. Wesley's journal shows that Benezet's work, and Lord Mansfield's deliberations in the case of Somerset, caused him much disquiet.
Two years later, in 1774, he wrote a tract called "Thoughts on Slavery" that went into four editions in two years. In it, he attacked the Slave Trade and the slave-trader with considerable passion and proposed a boycott of slave-produced sugar and rum. In August 1787, he wrote to the Abolition Committee to express his support. In 1788, when the abolition campaign was at its height, he preached a sermon in Bristol, one of the foremost slave trading ports. In those days, an anti-slavery sermon could not be preached without considerable personal risk to the preacher and a disturbance broke out.
He maintained an interest in the abolition movement until he died. Wesley also famously said:
"Give liberty to whom liberty is due, that is, to every child of man, to every partaker of human nature. Let none serve you but by his own act and deed, by his own voluntary action. Away with all whips, all chains, all compulsion. Be gentle toward all men; and see that you invariably do with every one as you would he should do unto you."