Martin Luther, "How Christians Should Regard Moses," trans. and ed. by E. Theodore Bachmann, Luther's Works: Word and Sacrament I, vol. 35 (Philadelphia: Muhlenberg Press, 1960), 161-174. This sermon was delivered on August 27, 1525, in Luther's long series of seventy-seven sermons on Exodus preached from October 2, 1524, to February 2, 1527. Either before or after you read this article, you may want to read "Comments on Luther's 'How Christians Should Regard Moses'."
Dear friends, you have often heard that there has never been a public sermon from heaven except twice. Apart from them God has spoken many times through and with men on earth, as in the case of the holy patriarchs Adam, Noah, Abraham, Isaac, Jacob, and others, down to Moses. But in none of these cases did he speak with such glorious splendor, visible reality, or public cry and exclamation as he did on those two occasions. Rather God illuminated their heart within and spoke through their mouth, as Luke indicates in the first chapter of his gospel where he says, “As he spoke by the mouth of his holy prophets from of old” [Luke 1:70].
Now the first sermon is in Exodus 19 and 20; by it God caused himself to be heard from heaven with great splendor and might. For the people of Israel heard the trumpets and the voice of God himself.