Generally the word ‘Nastik’ is translated into atheist, however it is not the correct translation as atheist is a person who does not belive in the existence of God whereas the word Nastik has a total different meaning.
The meaning of ‘Nastik’ is a person who does not believes in Veda(S) fully, here believing in Veda(S) means to accept them Apaurusheya (not written by any man) and Brahmaswarup (Bhagvan in literary form) and Nitya (with no beginning and end).
So even if a person believes in the existence of God like Christians, Muslims, Jews or even some organizations which appear to be Hindu but do not have utmost faith in the Veda(S) are Nastik, thus the term ‘Nastik’ and Atheist is far different.