After his conversion to evangelical Christianity, he gave up his racehorse, gambling and attendance at clubs. Although a serious young man, he was still fun to be with and, despite some of his friends thinking his new found belief was a madness, a childhood friend remarked, "If this be madness, I hope that it will bite us all!"
His new beliefs affected his public life. Before, he had usually voted with Pitt but now he was guided by his conscience. He and his evangelical friends were nicknamed "the Saints" by upper class circles but he won widespread respect. He championed many causes but it was the fight against the Slave Trade and slavery that he worked most tirelessly for. His interest was rekindled by a letter from Sir Charles Middleton, suggesting he should represent the cause in Parliament. William Pitt also encouraged him to take up the cause.
In early 1787, Thomas Clarkson called upon Wilberforce with a copy of his Essay on Slavery. This was the first time the two men had met, and a collaboration was formed which lasted over fifty years. The skills of the two men complimented each other. Wilberforce was able to turn the vague sentiment amongst the more privileged in society, into real opposition and rise above party politics to obtain support from many in Parliament.
From 1789, Wilberforce regularly introduced bills in Parliament to ban the Slave Trade. He was fiercely opposed by those making fortunes from the trade, who used all kinds of delaying tactics. The first time a bill was introduced, Wilberforce lost the debate by 163 votes to 88 but he never gave up. A bill to cease the trade was passed by the House of Commons in 1792 - but with the amendment that the ban should be 'gradual', which those with an interest in the trade interpreted as 'never'.