One who does not believe in the existence of God -

Atheist (noun):
English Meaning - One who does not believe in the existence of God or any Gods.
Bengali meaning - নাস্তিক।

অন্যান্য অপশনগুলো হলো - 
Theist (আস্তিক): A person who believes in the existence of god.
Fanatic (অতিশয় গোঁড়া ব্যক্তি ): A person who is extremely enthusiastic about some interest or activity. 
Pantheist (সর্বেশ্বরবাদী): One who believes in the presence of God in everything in the universe. 

Source: Merriam-Webster dictionary, Accessible Dictionary by Bangla Academy.