The Collaborative International Dictionary of English v.0.48:

Syncretist \Syn"cre*tist\, n. [Cf. F. syncr['e]tiste.]
   One who attempts to unite principles or parties which are
   irreconcilably at variance; specifically (Eccl. Hist.), an
   adherent of George Calixtus and other Germans of the
   seventeenth century, who sought to unite or reconcile the
   Protestant sects with each other and with the Roman
   Catholics, and thus occasioned a long and violent controversy
   in the Lutheran church.
   [1913 Webster]