Mercy and truth, etc. Here it is to be noted that (1) truth is sometimes contrasted with what is false and lying and (2) is sometimes contrasted with the shadow and the figure. In John 6:32 we read: “It was not Moses who gave you the bread from heaven, but My Father gives you the true bread from heaven.” Does this mean that the former bread was false? No, but it was figurative and shadowy. So it is with “I am the true vine” (John 15:1), namely, in opposition to the shadow of the natural vine or even of the false and lying vine. (3) Sometimes it is in opposition to vanity. Thus all is vanity with a man who seeks vain goods and flees vain evils, fears where there is no fear and loves where there is no love. In all these ways Christ is the Truth, for He is the fulfillment of all types and the true and firm salvation and faithful good, neither lying nor vain. (4) Truth, in a way that I think fits better to the theme, is called the faithful setting forth of what was promised over against reduction and omission of what was promised. Thus Christ is the Truth, because He showed forth the promise made to the fathers concerning Him, so that God might be true in His promises, since the salvation which He promised has been given.
Luther’s works, vol. 11: First Lectures 2