I just listened to Dr. White's recent evaluation of Real Catholic TV's video on the immaculate conception. When Roman Catholics find a Luther tidbit about Mary that seems to support Mariolatry, they run with it, even if the context contradicts the evidence they're using. This quote is being taken entirely out of context. It has nothing to do with Mary's immaculate conception. Rather than discussing Mary’s sinlessness, Luther's later writings insist Christ’s sinlessness was due entirely to the miraculous work of the Holy Spirit during His conception. The quote isn't about Mary's conception in her mother's womb, it's about Christ's conception in Mary's womb.
Here is the quote in context. In 1532 he preached:
Mary said, Behold the handmaid of the Lord; be it unto me according thy word.
14. That day, that moment when Mary assented to the angel Gabriel's announcement, Christ was conceived. In that hour when she said, "Be it unto me according to thy word," she conceived and became the mother of God; and Christ, therewith, became true God and true man in one person. Even though he is a tiny fetus, at that moment he is both God and man in Mary's womb, an infant, and Mary is the mother of God.
15. The Turks and the Jews make fun of this article of faith and feel that they have excellent reason to deride it. For that matter, we could banter about it as well as they. But as Christians, we must firmly hold onto this article of faith and never waver. From the beginning of time it has been prophesied that God's Son would become man and that his mother would be a virgin. The first prophecy given Adam and Eve soon after the fall (Gen. 3:15) stated: "And I will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shall bruise his heel." God does not say the seed of the man, but rather the seed of the woman. Therefore, the mother of this serpent crusher must be a virgin. Later the patriarchs and the prophets also prophesied of this, until finally the beloved apostles proclaimed it to all the world. We have been baptized into this faith and are called Christians because we believe and confess it to be true. Let us, therefore, persevere unwaveringly in this faith. And if, as time goes on, sectarian spirits deny it, let us take a staunch stand in behalf of it.
16. This article is really the bottom line. Christ wanted his beginning to be like ours, but without sin, because he wanted to sanctify us wholly. We begin life in sin, we are conceived in sin, born in sin, no matter whether we be emperor, king, prince, rich, or poor; every human being is conceived in sin according to Psalm 51:5. Only Christ has the distinction and the honor to have been conceived by the Holy Ghost's power. Since from our conception we are sinful, we are people whose flesh and blood and everything about us are soiled by sin, as indeed we see in ourselves; or when we look at those around us in the world, beset by evil desire, pride, multiple devils, and miserable unbelief. Thus we are conceived and born. For all of mankind is conceived and born in accord with creation's decree, as recorded (Gen. 1:28): `Be fruitful, and multiply, and replenish the earth." Christ could not be subject to such impure sinful conception and birth. He, indeed, was a genuinely true, natural human being, but not conceived or born in sin as all other descendants of Adam. That is why his mother had to be a virgin whom no man had touched, so that he would not be born under the curse, but rather conceived and born without sin, so that the devil had no right or power over him. Only the Holy Spirit was present to bring about the conception in her virgin body. Mother Mary, like us, was born in sin of sinful parents, but the Holy Spirit covered her, sanctified and purified her so that this child was born of flesh and blood, but not with sinful flesh and blood. The Holy Spirit permitted the Virgin Mary to remain a true, natural human being of flesh and blood, just as we. However, he warded off sin from her flesh and blood so that she became the mother of a pure child, not poisoned by sin as we are.
17. Thus what the angel spake came true: "He shall be great, and shall be called the Son of the Highest." For in that moment when she conceived, she was a holy mother filled with the Holy Spirit and her fruit is a holy, pure fruit, at once true God and truly man, in one person. In time, then, this godly mother gave birth to God's Son, a genuine man, but without any sin. Undoubtedly, his blood was red, his flesh, white; he suckled at his mother's breasts, ate porridge, cried, and slumbered like any other child; but his flesh and blood were holy and pure. He is a holy person, the son of a pure virgin and God's Son, true God and man in one person. [Sermons of Martin Luther Vol. 7 (Grand Rapids: Baker Books, 2000), pp. 291-293].
In 1534 Luther explained that Christ was “born of a young maiden, as you and I are born of our mothers. The only difference is that the Holy Spirit engineered this conception and birth, while in contrast we mortals are conceived and born in sin.”[Ibid., 294.]. As Jaroslov Pelikan has noted, Mary functioned in Luther’s theology as “the guarantee of the reality of the incarnation and of the human nature of Christ.”