I was struck by the following passage from Dr. Luther's Lectures on Galatians (CPH 1963), re: Gal. 4:8-9. It appears to shed some additional light on his comments concerning the First Commandment in the Large Catechism (the object of some controversy in recent years):
"Whoever falls from the doctrine of justification is ignorant of God and is an idolater. Therefore it is all the same whether he then returns to the Law or to the worship of idols; it is all the same whether he is called a monk or a Turk or a Jew or an Anabaptist. For once this doctrine is undermined, nothing more remains but sheer error, hypocrisy, wickedness, and idolatry, regardless of how great the sanctity that appears on the outside.
"The reason is this: God does not want to be known except through Christ; nor, according to John 1:18, can He be known any other way. Christ is the Offspring promised to Abraham; on Him God founded all His promises. Therefore Christ alone is the means, the life, and the mirror through which we see God and know His will.
"Through Christ God announces His favor and mercy to us. In Christ we see that God is not a wrathful taskmaster and judge but a gracious and kind Father, who blesses us, that is, who delivers us from the Law, sin, death, and every evil, and endows us with righteousness and eternal life through Christ. This is a sure knowledge of God and a true divine conviction, which does not deceive us but portrays God Himself in a specific form, apart from which there is no God."
We have heard St. Paul proclaim, this past week, that in Christ all the fullness of Deity dwells in bodily form. That seems to be at the heart of Dr. Luther's comments here, and as he elsewhere sings: "For us fights the valiant One, whom God Himself elected. He’s by our side upon the plain with His good gifts and Spirit. Ask ye, Who is this? Jesus Christ it is, of Sabaoth Lord, and there’s none other God; He holds the field forever."