Interpreting the Psalmist’s cry for understanding, Luther discusses the need for understanding to grow over time: “The Psalmist prays for an understanding against the mere letter,for the Spirit is understanding. For as the years have passed, so has the relationship grown closer between letter and the Spirit. For what was a sufficient understanding in times past, has now become the letter to us. Thus at the present time, as we have said, the letter itself is more subtle in nature than before. And this is because of the progress of time. For everyone who travels, what he has left behind and forgotten is the letter, and what he is reaching forward to is the Spirit. For what one already possesses is always the letter, by comparison with what has to be achieved.” In the conflict with Arius, the doctrine of the Trinity “was the Spirit,” but in Luther’s time it had become “the letter, because it is something publicly known.” It ceases to be letter only when “we add something to it, that is, a living faith in it. Consequently we must always pray for understanding, in order not to be frozen by the letter that kills.”