Atheists; what do you base your morals on?

[Neo-Scholastic]

The Bible is very clear in it's advocacy of slavery, both in the old and new testament. So John Wesley's position was not that of a practicing Christian...

Completely false. The Bible does not advocate slavery. No where does it say to go out and enslave others. It does however regulate an existing evil to make it more humane. Considering that over 1/3 of people in the ancient world were slaves, the practice wouldn't disappear over-night. God was fully aware that change would take a long time and advised people accordingly.