someone who does not believe the doctrine of free will
necessarian
Of or pertaining to necessity or necessitarianism: opposed to libertarian. The Arminian has entangled the Calvinist, the Calvinist has entangled the Arminian, in a labyrinth of contradictions. The advocate of free-will appeals to conscience and instinct — to an a priori sense of what ought in equity to be. The necessitarian falls back upon the experienced reality of facts. Froude, Calvinism. (The Century Dictionary, The Century Co., New York, 1911).