“However, we do not let ourselves be troubled by the blasphemies which the devil, through the mouths of his lying servants, speaks against Christ the Lord—now against His divinity, now against His humanity—and by the attacks which he then makes against Christ’s office and work. But we cling to the Scriptures of the prophets and apostles, who spoke as they were moved by the Holy Spirit (2 Peter 1:21). Their testimony about Christ is clear. He is our Brother; we are members of His body, flesh and bone of His flesh and bone. According to His humanity, He, Christ, our Savior, was the real and natural fruit of Mary’s virginal womb (of which Elizabeth, filled with the Holy Spirit, said to her in Luke 1:42: “Blessed is the fruit of your womb!”). This was without the co-operation of a man, and she remained a virgin after that.* Everything else that a mother imparts to a child was imparted by Mary, the mother of God’s eternal Son. Even the milk He sucked had no other source than the breasts of this holy and pure mother” (Luther’s Works, vol. 22, p.22-23).
Note: Luther’s private opinion is that Mary was perpetually a virgin.