The EEOC just released its Compliance Manual on Religious Discrimination and lack of religious faith is protected from discrimination at the workplace.
You hear that? Atheists & agnostics - you matter too!
Here is what the manual states:
Definition of Religion
Comment: Some commenters expressed concern that the draft did not make sufficiently clear that Title VII protects against discrimination based on a lack of religious faith.
Response: The Commission has made additions to reference repeatedly that discrimination based on a lack of religious faith is prohibited.