Atheism or secularism, they have a very similar purview, but where the atheist only says what he is against, the secular advocate implies a legal distinction. It doesn’t definitively say that he is an atheist, only that he cares about the separation of church and state. In fact, many secular activists have been theists despite what many think. Some of our founding fathers were diligent theists and loosely related to the Quaker movement. This can be seen on every package of Quaker Oats, drawings of George Washington and the rest of the authors of the Constitution show them as having hair and garb similar to that of the Quaker Oats mascot. The Quakers were deeply religious and so the founding fathers can be reasoned as also being deeply religious.
The point is that atheists are basically just against religion. A secularist or someone devoted to secularism is more of a scholar intent on promoting secular values and societies, often in an attempt at preserving religious freedoms. “Atheist” and “atheism” both are negatively linked with communism whereas “secular” is not; it is inextricably linked with positivity and the nature of equality.