There were not a lot of publicly avowed atheists in those days. The fight against slavery was largely led by Christians such as evangelical William Wilberforce in England. Of course, there were self-described Christians on both sides of the Civil War in this country. However, there can be little doubt that the altruism inspired by Christianity was crucial to abolition of slavery. Slavery continues in the Muslim world to this day.
Incidentally, can you name how many hospitals, schools, and other philanthropic works were founded by atheists? There are hundreds, if not thousands, founded by Christians, both Catholic and Protestant, including most of the elite Ivy League schools, now bastions of the secular left.
While I would would not in any way want to denigrate the wonderful philanthropy of Christians, it is probably worth remembering that the two richest men in our country, both of whom have given or pledged a large fraction of their fortunes to charity, are atheists or agnostics. The Bill and Melinda Gates Foundation does wonderful work in education and health.