Both expressions  are used and the [-ist][1] suffix is also to indicate someone who advocates a belief, as in this case: 

[Secularist][2] 

> - an advocate of secularism; someone who believes that religion should be excluded from government and education. 

(The Free Dictionary)


To avoid confusion I used the expression *secularists are* whose usage  appear to be increasing: 

[Ngram][3]: *secular person* vs *secularist is* vs *secularists are* 


  [1]: http://www.thefreedictionary.com/-ist
  [2]: http://www.thefreedictionary.com/secularist
  [3]: https://books.google.com/ngrams/graph?content=secular%20person%2Csecularist%20is%2Csecularists%20%20are&year_start=1800&year_end=2008&corpus=15&smoothing=3&share=&direct_url=t1%3B%2Csecular%20person%3B%2Cc0%3B.t1%3B%2Csecularist%20is%3B%2Cc0%3B.t1%3B%2Csecularists%20are%3B%2Cc0