c. 1300, "a change or turn from one religion to another," especially to Christianity, from Old French convertir "to turn around, turn towards; change, transform; convert, win over," from Vulgar Latin *convertire, from Latin convertere "turn around, transform," from assimilated form of com "with, together" (see con-) + vertere "to turn" (from PIE root *wer- (2) "to turn, bend").
The Latin verb was glossed in Old English by gecyrren, from cierran "to turn, return." General sense of "change into another form or substance, transmute" is from late 14c. Transitive sense of "turn from one use or destination to another" is from late 15c. Related: Converted; converting.
1560s, "person whose faith has been changed from one religion to another," from convert (v.). Earlier was convers (early 14c.), from Old French converse (n.). General (non-religious) sense of "person converted from one opinion or practice to another" is from 1640s.