Wherever in Scripture you find God speaking about God, as if there were two persons, you may boldly assume that three Persons of the Godhead are there indicated. Thus in the passage under discussion we hear the Lord say that the Lord will build a house for David. Likewise we read in Gen. 19:24: “Then the Lord rained on Sodom and Gomorrah brimstone and fire from the Lord out of heaven.” For the Holy Spirit is no fool or drunkard, who would speak one iota, much less a word, in vain. If the Lord, that is, the Son rains fire and brimstone from the Lord, that is, the Father, the Holy Spirit is simultaneously present. It is He who speaks these words by Abraham, or whoever it might be, about the two Lords. And still these three are one Lord, one God, who rains fire and brimstone.
Luther’s Works Vol. 15
TREATISE ON THE LAST WORDS OF DAVID
2 Samuel 23:1–7
Translated by MARTIN H. BERTRAM