Votary

votary [VOH-tuh-ree]
n. a person who is bound by solemn religious vows, as a monk or a nun

[From Latin vōtum, (vow, "a solemn promise, pledge, or personal commitment")]

The etymological roots of devotion are the same as in votary and vow. Essentially, a votary is a person devoted to God.

In a recent article in Slate.com, the noted atheist Christopher Hitchens expressed his displeasure at the secular government of Pakistan for handing control of a part of that country over to a group of medieval men devoted to imposing Islamic law, as they interpret it, on the modern people of the Swat Valley:

In a wishful attempt to bring peace with the Taliban in Pakistan itself, the government has recently ceded a fertile and prosperous and modernized valley province—the former princedom of Swat—to the ultraviolent votaries of the one party and the one God. This is not some desolate tribal area where government and frontier have been poorly delineated for decades, as in Waziristan. It is a short commute from the capital city of Islamabad.