Christians: If Christianity is true why are there all these other religions?

Remember when people all thought it was cool to own slaves? Society after society held slaves. People captured slaves. People sold slaves. People kept slaves in shackles. Now remember when society started thinking of slavery as wrong? Why would there be society upon society that believed it was OK, but we now know we were mistaken? Should we decide that freedom is stupid and slavery is correct because so many societies practiced it? Sometimes people are just wrong.