Luther

Where there is not the people of God, there are not the keys either. For Christ has left them behind so that there shall be a public symbol and sanctuary where the Holy Spirit can sanctify sinners and where Christians can bear witness that they are one holy people under Christ in this world. And those who will not turn away from their sins and be sanctified must be expelled from this holy people—that is, bound and locked out by the keys.—On the Councils and the Churches, 1539