Since atheists don't listn to anything other than what they think, I'm sure it's a waste of time to explain why God allows slavery. But I want to let them know so they have no more justification to distort the bible even though they'll distort it anyway.

God allows slavery for many reasons, the chief of which is to show:

1) "The greatest among you will be your servant"
2) The relationship that mankind has to have to God
3) That happiness comes from the inside, regardless of outside circumstances
4) That man doesn't dictate his own fate
5) How to treat others who treat us badly
6) Why slaves are exalted in the eyes of God and proud men aren't
7) To learn humility
8) That even though all men aren't equal in the eyes of men, they are equal in the eyes of God

Those are just some reasons that God allows slavery. But since atheists aren't here to learn and understand Christianity, as they claim, then they will continue to look for reasons to keep hating God. They will thus distort the above arguments. They don't understand that if a man buys another human being and treats him badly, then he is indeed showing that his servant is greater than his master!

And most importantly, God allowed slavery to foreshadow the coming of Christ who would be a servant to the world and yet be treated as badly slaves are treated by their wicked slavemasters.

But since atheists don't understand simple concepts like those above, then all they can see is the justification of slaves to oppress their masters just like their masters opporess them. But God is teaching us spiritual concepts which are completely lost on atheists. Only a desire to understand God will make the above list clear to them. But the desire to hate God will keep them stuck in distorting the bible and passing along false claims about God.