Now let us look at the words themselves: “I AM THE GODS OF ABRAHAM.” For in the Hebrew it is אֱלֹהִים, which in the plural number denotes Gods, just as above in chapter 1:1: “In the beginning God created, etc.” We Christians must have no doubt about the oneness of God. But the nature of that oneness is inscrutable to reason and human wisdom, for it is a revelation of the Holy Spirit and of faith. The Jews and the Turks laugh at us and attribute a most impudent lie to us, namely, that we set up a plurality of gods and believe that there are three gods. Therefore they boast that they are the people of God and that they are honored with many brilliant victories and with the wealth of the entire world because they believe in one God, but that we have been afflicted with so many defeats because we invent more gods. For this reason they call us idolaters and horribly blaspheme the Son of God, as all their histories bear witness. Therefore our hearts must be fortified against these blasphemous utterances and this boast, which, as it appears to reason, is altogether too true and plausible. For it is completely certain that we believe in a God who is completely one and completely simple.62 But the fact that the Turk does not understand our faith or doctrine properly and does not hear who that one and completely uncompounded God is—this is his own fault, not ours. For we teach and believe not only that there is one God, but that He is completely simple and completely one in His state of being one. We do not separate these three: the Father, the Son, and the Holy Spirit. We do not make separate gods. No, we believe in God who is completely one and completely simple.
Luther’s works, vol. 5: Lectures on Genesis: Chapters 26-30