Care to name some of these atheist abolitionists? I can easily name 5 prominent Christian abolitionists: John Newton, William Wilberforce, Elijah Lovejoy,  Harriet Tubman, Frederick Douglass, Harriet Beecher Stowe, William Lloyd Garrison, and that's just off the top of my head.

You can't divorce the abolitionist movement from Christianity anymore than I could divorce it from the enlightenment.