Phoo~ all potentially explosive words these days. Lets see if we can help you make a distinction: 1) Atheist- A person who does not believe in religion, and may express a non-belief in gods of any religion. 2) Infidel- This is an accusative description from one religion to all that are not of the same religion. It could also be leveled at an atheist, or a person of other religions. A bit closer to "heretic" in my opinion. 3) Pagan- Another accusative description, but a bit out dated. It used to mean people who believe in other religions (or practise other forms of worship) that is not from the larger religious groups. Possibly fringe groups. 4) Freethinker- A less accusative description for a person who does not believe in religion, but has a more neutral tone to it. Other words such as "apostate", "skeptic", "agnostic", "misbeliever" also exist.

Darryl's answer is quite odd - some of it is correct, but most of it reads like something out of a dictionary. Infidel is very accusative, and these days is mostly used (at least in Hollywood movies!) by Muslims to refer to anyone else. Pagan isn't accusative, or at least hasn't been for a century or more, and also doesn't really mean a non-believer as such. Pagans tend to believe in 'old gods', or perhaps natural gods. They were only seen as non-believers by Christians in England historically. I've never heard anyone use the phrase 'freethinker', and I've never heard of misbeliever either. Agnostic is a better word - it's less strong than Atheist. An agnostic thinks there isn't a god. An atheist knows there isn't one! Non-believer is a much more neutral term to use.