The Collaborative International Dictionary of English v.0.48:

Redemptionist \Re*demp"tion*ist\, n. (R.C.Ch.)
   A monk of an order founded in 1197; -- so called because the
   order was especially devoted to the redemption of Christians
   held in captivity by the Mohammedans. Called also
   Trinitarian.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Trinitarian \Trin`i*ta"ri*an\, a. [Cf. F. trinitaire. See
   Trinity.]
   Of or pertaining to the Trinity, the doctrine of the Trinity,
   or believers in that doctrine.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Trinitarian \Trin`i*ta"ri*an\, n.
   1. One who believes in the doctrine of the Trinity.
      [1913 Webster]

   2. (Eccl. Hist.) One of a monastic order founded in Rome in
      1198 by St. John of Matha, and an old French hermit, Felix
      of Valois, for the purpose of redeeming Christian captives
      from the Moslems.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Trinitarian
    n 1: adherent of Trinitarianism