What is a Christian response to the claim that atheists make that "the Bible condones slavery" in Colossians 3:22-25?

Colossians 3:22-25 (NIV) states: > 22 Slaves, obey your earthly masters in everything; and do it, not only when their eye is on you and to curry their favor, but with sincerity of heart and reverence for the Lord. 23 Whatever you do, work at it with all your heart, as working for the Lord, not for human masters, 24 since you know that you will receive an inheritance from the Lord as a reward. It is the Lord Christ you are serving. 25 Anyone who does wrong will be repaid for their wrongs, and there is no favoritism. Atheists on a certain Internet forum used this as an argument that the Bible condones slavery. What is a Christian counterargument to this statement?