Why do people say anyone who doesn't believe in god will go to hell?

Some people that I know say that if you do not worship god you will go to hell. But aren't we all allowed do believe in what we want? I don't think it is right to say that to someone. I believe in god but I don't go to church that often and some of my classmates tell me that that's dishonoring god and that he will not love me... isn't that mean?