Slavery was very much considered a part of the natural order in the ancient Mediterranean. Philosophic texts are our best source for this and the closest any philosopher came to opposing slavery was Alcidamas, who only posed that enslavement was not the natural state of man. Extremely mainstream philosophers like Aristotle wrote that slavery was natural and even essential. It was a hierarchical world. There were zero large-scale movements to end the practice.

There were large-scale slave revolts, such as the Spartacus uprising in 73 BC, but I find it doubtful that their end goal was to end the institution of slavery, but rather just their own.

Nearly all slaves in the ancient world were slaves because either they or their ancestors had lost a war or battle. Slavery, then, was not understood as a systemic injustice, but a consequence of war. The Spartacus rebels and other slave uprisings in the Roman world were not fighting a system of oppression, but rather trying to win back through war what they had lost through war, their freedom.