Defining the marks of a true church was an interest of the Reformers for obvious reasons. The objective of this exercise was to find a set of criteria that can be used to distinguish true churches from false churches (or dead churches in our day). Martin Bucer contributed to this discussion through his commentary on Paul’s letter to the Ephesians. His five marks of the true church were

• heeding the shepherd's voice
• the ministry of teaching
• suitable ministers of the Word
• the lawful dispensation of the sacraments
• righteousness and holiness of life

This lists seems to reflect a reaction against the current state of the church that Bucer had just left. The priests often lived far from moral lives. Many could not read a word of Latin so not only were the people prevented from hearing teaching in the common language, but they were subjected to mumbled Latin. I also find it interesting that correct doctrine was not explicitly stated as a mark.

Reference: “Bucer’s Commentaries on Ephesians” by Peter Stephens