If they aren't meeting with others of the same disbelief regularly and taking classes on how to spread their disbelief better, becoming evangelical and extremely like churchgoers?
Then they would be atheist (small a)...
If they are attending meetings, becoming evangelical, carrying tracts (I've seen them), creating anti-religion websites to "disabuse" those who believe of "incorrect notions" then they become Atheist (big a)... that one is a religion. It takes exactly the same amount of faith to be absolutely sure that there is no God as there does to believe that there is one. When one wants to spread their Faith all over the rest of us they are definitely religious.