This psalm was composed for answering. In this word the Spirit calls us to understand the mystery of this matter which we are constantly dealing with, namely, that we sing and pray in church by turns. For what does it mean for one to answer to the other, if not to help complete the same song or the same work and bring it to a perfect end? So also, says the apostle, I may complete the sufferings of Christ which are lacking in my flesh (Col. 1:24). This is what it means to suffer together for the purpose of reigning together, to endure together for the purpose of reigning together (2 Tim. 2:12).
Luther’s works, vol. 11: First Lectures 2