The British initially became involved in the slave trade during the 16th century. By 1783, the triangular route that took British-made goods to Africa to buy slaves, transported the enslaved to the West Indies, and then brought slave-grown products such as sugar, tobacco, and cotton to Britain, represented about 80 percent of Great Britain’s foreign income.  British ships dominated the trade, supplying French, Spanish, Dutch, Portuguese and British colonies, and in peak years carried forty thousand enslaved men, women and children across the Atlantic in the horrific conditions of the middle passage.
Of the estimated 11 million Africans transported into slavery, about 1. 4 million died during the voyage.  The British campaign to abolish the slave trade is generally considered to have begun in the 1780s with the establishment of the Quakers’ antislavery committees, and their presentation to Parliament of the first slave trade petition in 1783.  The same year, Wilberforce, while dining with his old Cambridge friend Gerard Edwards, met Rev.
James Ramsay, a ship’s surgeon who had become a clergyman on the island of St Christopher (later St Kitts) in theLeeward Islands, and a medical supervisor of the plantations there. What Ramsay had witnessed of the conditions endured by the slaves, both at sea and on the plantations, horrified him. Returning to England after fifteen years, he accepted the living of Teston, Kent in 1781, and there met Sir Charles Middleton, Lady Middleton, Thomas Clarkson, Hannah More and others, a group that later became known as the Testonites.
Interested in promoting Christianity and moral improvement in Britain and overseas, they were appalled by Ramsay’s reports of the depraved lifestyles of slave owners, the cruel treatment meted out to the enslaved, and the lack of Christian instruction provided to the slaves.  With their encouragement and help, Ramsay spent three years writing An essay on the treatment and conversion of African slaves in the British sugar colonies, which was highly critical of slavery in the West Indies.
The book, published in 1784, was to have an important impact in raising public awareness and interest, and it excited the ire of West Indian planters who in the coming years attacked both Ramsay and his ideas in a series of pro-slavery tracts.  Diagram of a slave ship, the Brookes, illustrating the inhumane conditions aboard such vessels Wilberforce apparently did not follow up on his meeting with Ramsay.  However, three years later, and inspired by his new faith, Wilberforce was growing interested inhumanitarian reform.
In November 1786 he received a letter from Sir Charles Middleton that re-opened his interest in the slave trade.  At the urging of Lady Middleton, Sir Charles suggested that Wilberforce bring forward the abolition of the slave trade in Parliament. Wilberforce responded that “he felt the great importance of the subject, and thought himself unequal to the task allotted to him, but yet would not positively decline it”. 
He began to read widely on the subject, and met with the Testonites at Middleton’s home atBarham Court in Teston in the early winter of 1786–87. 62] In early 1787, Thomas Clarkson, a fellow graduate of St John’s, Cambridge, who had become convinced of the need to end the slave trade after writing a prize-winning essay on the subject while at Cambridge, called upon Wilberforce at Old Palace Yard with a published copy of the work.  This was the first time the two men had met; their collaboration would last nearly fifty years.  Clarkson began to visit Wilberforce on a weekly basis, bringing first-hand evidence he had obtained about the slave trade.
The Quakers, already working for abolition, also recognised the need for influence within Parliament, and urged Clarkson to secure a commitment from Wilberforce to bring forward the case for abolition in the House of Commons.  It was arranged that Bennet Langton, a Lincolnshire landowner and mutual acquaintance of Wilberforce and Clarkson, would organize a dinner party in order to ask Wilberforce formally to lead the parliamentary campaign.  The dinner took place on 13 March 1787; other guests included Charles Middleton, Sir Joshua Reynolds, William Windham, MP, James Boswell and Isaac Hawkins Browne, MP.
By the end of the evening, Wilberforce had agreed in general terms that he would bring forward the abolition of the slave trade in Parliament, “provided that no person more proper could be found”.  The same spring, on 12 May 1787, the still hesitant Wilberforce held a conversation with William Pitt and the future Prime Minister William Grenville as they sat under a large oak tree on Pitt’s estate in Kent.  Under what came to be known as the “Wilberforce Oak” at Holwood, Pitt challenged his friend: “Wilberforce, why don’t you give notice of a motion on the subject of the Slave Trade?
You have already taken great pains to collect evidence, and are therefore fully entitled to the credit which doing so will ensure you. Do not lose time, or the ground will be occupied by another. “ Wilberforce’s response is not recorded, but he later declared in old age that he could “distinctly remember the very knoll on which I was sitting near Pitt and Grenville” where he made his decision.  Wilberforce’s involvement in the abolition movement was motivated by a desire to put his Christian principles into action and to serve God in public life.
He and other Evangelicals were horrified by what they perceived was a depraved and unchristian trade, and the greed and avarice of the owners and traders.  Wilberforce sensed a call from God, writing in a journal entry in 1787 that “God Almighty has set before me two great objects, the suppression of the Slave Trade and the Reformation of Manners [moral values]”.  The conspicuous involvement of Evangelicals in the highly popular anti-slavery movement served to improve the status of a group otherwise associated with the less popular campaigns against vice and immorality.