Marks of the Church
How you can tell where the true church is, given that it is not identified with any institution. The Augsburg Confession names two: the gospel purely preached and the sacraments rightly administered. The late Luther, in On the Councils and the Church, lists seven, adding baptism, the Supper, the keys, the ministry, public prayer, and being under the cross. Notably, none of them is a building.
In Church and ministry — Polity, offices, and the ELCA's relationships.