Votary

Votary \VOH-tuh-ree\ , noun;
1. One who is devoted, given, or addicted to some particular pursuit, subject, study, or way of life
2. A devoted admirer
3. A devoted adherent of a religion or cult
4. A dedicated believer or advocate

This word dates to the 1540's as "one consecrated by a vow" from Latin votum, which derives from the Proto-Indo-European base *ewegwh- ("to speak solemnly, vow"). Originally it referred to a monk or a nun, but by the 1590's it had taken on a more general sense of "ardent devotee of some aim or pursuit." Shakespeare used it in reference to love.