I think you have to be smart about protecting your own interests. If you want to keep your job. It would be difficult if they asked you directly, if it is a work place where they discriminate against atheism. Or even just make it more uncomfortable for you to work in that environment.
It very much depends on your circumstances at work and if you think you will be accepted or rejected for your world view.
From my understanding, the basis of Christianity, is that Jesus was about love. He apparently talks in the Bible (I was brought up half in an India Kult as Theist and half atheist, by Hippy Anglo Saxon English Parents), about loving thy neighbour and loving thy enemy. So really if you wanted to get technical with the Christian co workers, you could address their faith, if they treat you any differently due to your differing world view then they are directly going against their faith of loving there neighbours and their enemies. Jesus was about love man! What's with all the name calling / heavy judgement / avoidance strategies???
The other reason that Christians might shun atheist colleagues is due to fear. Religious people tend to be scared of atheists because I reckon they get it confused with anarchist, or something, like an atheist is unpredictable. Perhaps it would be useful, if you do end up disclosing your atheisms, so qualify your position with references regarding your ability to distinguish behaviour that might harm others, with behaviour that is caring of others :) - you could also point out that Jesus was in fact a nonviolent anarchist - aiming to overturn the domination,oppression and abuse of his people, by the Romans, through practicing and advocating nonviolent resistance to the oppressive behaviour of the hegemonic culture.