If Christianity is correct... how are those predating christianity judged?

Christian theology has a concept called "general revelation" based on Romans Chapter One. To make a long story short, and to oversimplify, people are judged on how they respond to whatever knowledge God gives them. It's still important to share Jesus with others, because it's better for people to walk in the light than to stumble around in the darkness. It still has to do with what God reveals, not what "society" teaches. I'm sure you can google "general revelation" and find more in-depth stuff.