In Reformed Theology, "the marks of a true church" are often delineated, to the effect of:
- Church discipline
- Administration of the sacraments
(These are the marks often denoted in systematic theology, but let it be noted that they are not intended to exclude other important marks such as suffering and love.)
A slightly altered formula is seen in Westminster Confession of Faith, Chapter 25 Of the Church, paragraph 4:
This catholic Church hath been sometimes more, sometimes less visible. And particular Churches, which are members thereof, are more or less pure, according as the doctrine of the gospel is taught and embraced, ordinances administered, and public worship performed more or less purely in them.
I found something similar in Calvin's letter to Cardinal Sadoleto (available in Vol. III of Banner of Truth's Tracts and Letters Calvin volumes, and in a slim booklet edited by John Olin entitled A Reformation Debate, page 63):
Since there are three things on which the safety of the Church is fonded, viz., doctrine, discipline, and the sacraments, and to these a fourth is added, viz., ceremonies, by which to exercise the people in offices of piety...
Calvin's letter of 1539 predates the Confession by a little over a hundred years, and is the earliest document that I have personally found this doctrine in. However, I find it unlikely that Calvin wrote any new doctrines in his letter to the Cardinal, so I'm wondering where the origin of this doctrine was.
Does Calvin have earlier writings on it? Did earlier Reformers such as Luther, Zwingli or Bucer discuss it? Were there pre-Reformation theologians who discussed it?