'Convert' is Latin for 'turn around completely' — religious conversion as a total turning of the soul.
To change the form, character, or function of something; to cause someone to change their religious faith or other beliefs (verb); a person who has changed their religious faith or other beliefs (noun).
From Latin convertere (to turn around, to transform completely), composed of con- (together, completely, expressing thoroughness) + vertere (to turn), from PIE *wer- (to turn, to bend). The prefix con- here functions as an intensive rather than a reciprocal prefix — convertere is not to turn mutually but to turn all the way around, to undergo complete transformation. The religious sense of turning to a new faith appeared early in Latin Christian writing
The word 'convertible' — as in a car with a retractable roof — literally means 'able to be turned around.' The automotive sense dates from the 1910s, when it described a car body that could be transformed between an open and closed configuration, perfectly preserving the Latin sense of 'turning into something else.'