Christianity, not Atheism, supported slavery.

That requires more nuance.

Take this, on one hand:

Although many Enlightenment philosophers opposed slavery, it was Christian activists, attracted by strong religious elements, who initiated and organized an abolitionist movement. [1] Throughout Europe and the United States, Christians, usually from 'un-institutional' Christian faith movements, not directly connected with traditional state churches, or "non-conformist" believers within established churches, were to be found at the forefront of the abolitionist movements.

for sources

On the other hand, you have this enlightened Enlightenment figures who did support slavery/racist policies & views:

• David Hume

• Immanuel Kant

• Thomas Hobbes

• John Locke

• Edmund Burke

and probably more.

I am very critical of the Christian church, and in general I believe that traditional religious dogmas are wholesale incompatible with modern morality. But.... I think you have it wrong in this particular case.