I think it's inaccurate to assume that people thought those of different races weren't equal by default.
Attitudes by various groups on other groups of people aren't uniform and have changed throughout time and place.
Whether one group thought the other group was inferior varied but usually hinged on technological advancement, cultural philosophies, and so on.
For example, European explorers thought native Americans were inferior because Native Americans did not build large cities, or roads, or clear large swaths of forest to farm, and hunted rather than raised livestock.
Native Americans thought the first Europeans they had contact with were inferior because they didn't know how to hunt, grow corn, or survive off the land.