The sale of indulgences was a practice where the church acknowledged a donation or other charitable work with a piece of paper (an indulgence), that certified that your soul would enter heaven more quickly by reducing your time in purgatory.

Luther sparked the Reformation in 1517 by posting, at least according to tradition, his "95 Theses" on the door of the Castle Church in Wittenberg, Germany - these theses were a list of statements that expressed Luther's concerns about certain Church practices - largely the sale of indulgences, but they were based on Luther's deeper concerns with Church doctrine.

but none of these efforts successfully challenged Church practice until Martin Luther's actions in the early 1500s.

An oligarchy is a system of political power controlled by a select group of individuals, sometimes small in number but it could also include large groups. For the Greeks (or more particularly the Athenians) any system which excluded power from the whole citizen-body and was not a tyranny or monarchy was described as an oligarchy. Oligarchies were perhaps the most common form of city-state government and they often occurred when democracy went wrong.