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

Secularist

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

(The Free Dictionary)

Secularist is also an adjective so to avoid confusion I used the expression secularists are whose usage appear to be increasing:

Ngram: secular person vs secularist is vs secularists are

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

Secularist

• 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: secular person vs secularist is vs secularists are