It doesn't make sense, Christianity?

Yes, I'm picking on Christianity because it is the religion I am most familiar with. (and that's not much) I have been wondering, if there were people before Christianity, which there obviously were, what happened to the people before Christianity came about? (or any religion for that matter) I hope I'm asking this correctly, so Jesus comes down to an already inhabited earth, where people have already died, long before Christianity was in existence. What happened to those people? "Sorry, haven't sent the son down yet, go on in"..? I clearly am not a Christian but I have always wondered what Christians have to say about this. Don't get defensive, it's a real question that has a real answer. If Christianity (and again any other religion) is a true religion, shouldn't it have been made clear from the beginning. Or is this another way to test your faith? Insightful answers are always appreciated.