Cowper helped Newton with his religious services and on his tours to other places. They held not only a regular weekly church service but also began a series of weekly prayer meetings, for which their goal was to write a new hymn for each one. They collaborated on several editions of Olney Hymns, which achieved lasting popularity. The first edition, published in 1779, contained 68 pieces by Cowper and 280 by Newton. The origin of the melody is unknown.
Most hymnals attribute it to an early American folk melody.
Newton was not only a prolific hymn writer but also kept extensive journals and wrote many letters. Historians accredit his journals and letters for much of what is known today about the eighteenth century slave trade. In Cardiphonia, or the Utterance of the Heart, a series of devotional letters, he aligned himself with the Evangelical revival, reflecting the sentiments of his friend John Wesley and Methodism. In 1780 Newton left Olney to become rector of St. Mary Woolnoth, St. Mary Woolchurch, in London. There he drew large congregations and influenced many, among them William Wilberforce, who would one day become a leader in the campaign for the abolition of slavery.