The troops composing this cordon were called Cossacks; but these were not the "Free Cossacks" best known to history and romance.

These latter lived beyond the frontier on the debatable land which lay between the two hostile races, and there they formed self-governing military communities.

Each one of the rivers flowing southwards--the Dnieper, the Don, the Volga, and the Yaik or Ural--was held by a community of these Free Cossacks, and no one, whether Christian or Tartar, was allowed to pass through their territory without their permission.
Officially the Free Cossacks were Russians, for they professed to be champions of Orthodox Christianity, and--with the exception of those of the Dnieper--loyal subjects of the Tsar; but in reality they were something different.

Though they were Russian by origin, language, and sympathy, the habit of kidnapping Tartar women introduced among them a certain admixture of Tartar blood.

Though self-constituted champions of Christianity and haters of Islam, they troubled themselves very little with religion, and did not submit to the ecclesiastical authorities.
As to their religious status, it cannot be easily defined.