c. 1300, "man consecrated to service in the Christian Church, an ecclesiastic;" also "an agent acting for a superior, one who acts upon the authority of another," from Old French menistre "servant, valet, member of a household staff, administrator, musician, minstrel" (12c.) and directly from Latin minister (genitive ministri) "inferior, servant, priest's assistant" (in Medieval Latin, "priest"), from minus, minor "less," hence "subordinate" (from PIE root *mei- (2) "small") + comparative suffix *-teros. Formed on the model of magister (see master (n.)).
Minister views a man as serving a church; pastor views him as caring for a church as a shepherd cares for sheep; clergyman views him as belonging to a certain class; divine is properly one learned in theology, a theologian; parson, formerly a respectful designation, is now little better than a jocular name for a clergyman; priest regards a man as appointed to offer sacrifice. [Century Dictionary, 1895]
The political sense of "high officer of the state, person appointed by a sovereign or chief magistrate of a country as the responsible head of a department of the government" is attested from 1620s, from notion of "one who renders official service service to the crown." From 1709 as "a diplomatic representative of a country abroad." A minister without portfolio (1841, in a French context) has cabinet status but is not in charge of a specific department.