Here is a wonderful quote from Martin Luther, from a sermon he preached around the same general time he was working on the Large Catechism, in which we find that phrase (LC II.66) that has been misunderstood to say that Martin Luther taught that Turks, Jews and unbelievers worship and believe in the One, True God. Here is yet another definitive refutation of that error, from Luther himself. Putting it plainly, it is the clear teaching of Holy Scripture: Know Chist, Know God. No Christ, No God.

"We should also note in this text how Christ refers to himself as being the one Man through whom the Father must be glorified. By this he also wants to prevent his people, the Jews—even though they had the Law and the glorious worship of God—from boasting of their holiness, because not one of them had the power to glorify the Father or to bring anyone to the knowledge of righteousness before God. For if the glory and knowledge of God could have been made manifest through the Law, then Christ would not have had to come, preach, suffer, and die to glorify the Father. This should also teach us how we must properly seek and apprehend or deal with God. For (as stated) “to glorify the Father” is nothing else than that we come to recognize and know who he is, what he has in mind, and what our relationship is to him. Only through Christ can man come to such knowledge. For in no other way did he want to reveal himself, for anyone to see his heart and will, than in and through Christ. We see now in Christ nothing but sincere, unfathomable love and grace [gnade]. On the other hand, outside of him there is nothing but wrath and displeasure [ungnade]. In short, whoever seeks or desires to serve God other than in Christ does not find nor does he serve the true God. For this reason I have often said and warned that anyone who wants to be safe ought to beware of all lofty thinking and speculating, where God in His majesty is explored without means and his work, will, and counsel are probed to obtain hidden and specific revelations, etc., since they not only fail and deceive but also lead and plunge men into the abyss. In this way, condemned also are all doctrines and beliefs on earth: of the Jews, Turks, monks, of false saints, or fanatics, and whoever they are who want to serve God, attain grace, and rid themselves of sin through means other than through Christ the Lord, such as through their works, godliness, great devotion, spiritual thoughts, etc. For it is resolved that he will not reveal himself nor let himself be found outside that one Mediator, so that where Christ is not, there likewise is no true God or worship. But more of that later."

"These words are most forcibly directed against the Arians and all heretics, Jews, and non-Christians, who say and boast that they believe in only one God, who made heaven and earth. And because of this article they condemn us Christians as though we were advancing another God. Thus the Turk says: ‘We believe in God, who created heaven and earth–not the one who spoke in Horeb with Moses, but who spoke with Muhammad."

For He [Christ] wants to show that they do not know the true God, though they think and boast that they do. They do not discover who He is, nor do they know how He must be known: that He is the only true God, who sent Jesus Christ, and so on. This is as much to say: Whoever wants to find the only true God must seek him solely in Christ, for there is no other true God than the one who sent Christ. He no longer wants to be the one who spoke with Moses, but the one who sent Jesus Christ. Whoever does not possess Christ must also be lacking the true God, even if he already knows and believes that there is only one true God. For He does not believe in Him who sent Christ and through Him gives eternal life. For this reason the emphasis is on the word: "You" "That they may know You, who alone are the true God." Which "You?" You who have sent Jesus Christ. It is s though he were saying: "Jews and others also have only one God, as they think, bu they do not know You, You who are the only true God, because they do not know Jesus Christ, the one sent by you. And in the meantime they make a god who conforms to their thoughts, a god who truly is not God, but purely nothing. Thus you see that th word "only" is not used in the sense of setting Himself apart from the Father with respect to the divine essence (since the other words adequately prevent this) but simply because he weaves the two of them together–the Father and himself–yes, firmly links the Father to himself in contrast to all who fashion another God or seek Him elsewhere than in Christ."

Source:

Martin Luther
Das 16. und 17 Kapitel Joh. von dem Gebet Christi (1530)
St.L 8:744-843; WA 28:70-200
(Aland Hilfsbuch No. 333)

Translated by Erwin W. Koehlinger
Unpublished