This psalm has a new title, because it is described neither as a song nor as a psalm, but as a “testimony,”1 like some others later on. Why? Perhaps because by this very word the psalmist forbids us to understand it in a physical sense. We are not to take it as a sign for a thing and a testimony for the setting forth of works. For a testimony is spoken with reference to something else, like John’s testimony concerning Christ. Thus this psalm is a testimony concerning a future rose, that is, a prophecy and sign of the church, for the psalm speaks about it and in its person. Indeed, the psalm itself is a testimony of the church. Thereby it witnesses to itself and to all concerning exclusively future good things, since it possesses no temporal goods. The fact that the psalmist speaks “for instruction” certainly shows that the whole psalm is full of doctrine.
Luther’s works, vol. 10: First Lectures I