Is it ensure, insure, or assure?
Old English epistol, from Old French epistle, epistre (Modern French épitre), from Latin epistola "letter," from Greek epistole "message, letter, command, commission," whether verbal or in writing, from epistellein "send to," from epi "to" (see epi-) + stellein in its secondary sense of "to dispatch, send" from PIE *stel-yo-, suffixed form of root *stel- "to put, stand," with derivatives referring to a standing object or place (see stall (n.1)).
Also acquired in Old English directly from Latin as pistol. Specific sense of "letter from an apostle forming part of canonical scripture" is c.1200.
a composition in prose or poetry written in the form of a letter to a particular person or group