The Collaborative International Dictionary of English v.0.48:

Secularist \Sec"u*lar*ist\, n.
   One who theoretically rejects every form of religious faith,
   and every kind of religious worship, and accepts only the
   facts and influences which are derived from the present life;
   also, one who believes that education and other matters of
   civil policy should be managed without the introduction of a
   religious element.
   [1913 Webster]

WordNet (r) 3.0 (2006):

secularist
    n 1: an advocate of secularism; someone who believes that
         religion should be excluded from government and education