The Christian Leadership Initiative at the University of Mary builds an argument for the immaculate conception of its namesake—the doctrine that she was conceived without the stain of original sin—which apparently a lot of readers find wanting:
In the Litany of Mary, one title given for her is “Ark of the Covenant,” for as bearer of God’s Word this is what she is. From the Book of Exodus through the books of the prophets, the perfection and purity of all related to the Ark is asserted. Mary falls into this requirement: God was preparing a place to be in her, and God’s standards continue in this human Ark. It is both biblical and reasonable that it should be so.
Over in the combox thread at First Things, proponents of Immaculate Conception try to build a better argument (with varying degrees of success). Craig Payne cites the argument that always gets me: i.e., we believe this doctrine for the same reason we believe the books of Hebrews and Revelation belong in the canon of Scripture—because the Church, guided by the Holy Spirit, said that it was so. Protestants—for obvious reasons—tend to not accept this line of reasoning. On the other hand, Leroy Huizenga cites the authority of Martin Luther: “She [Mary] is full of grace, proclaimed to be entirely without sin…God’s grace fills her with everything good and makes her devoid of all evil…God is with her, meaning that all she did or left undone is divine and the action of God in her. Moreover, God protected her from all that might be hurtful to her” (Luther’s Works, ed. Lehmann; Philadelphia: Fortress Press, 1968, 43:40).
From reading Catholic polemics, I sometimes get the impression that Luther was basically a Catholic. Other doctrines he zealously defended throughout his life include the “real presence” of Jesus in the Eucharist, and the perpetual virginity of Mary.
However, David DePerro makes what is, to my mind, the most compelling argument:
What Catholics call the Allegorical sense of scripture was ratified by our Divine Lord on the road to Emmaus, when he showed how the entire Old Testament pre-figured him. That is the Allegorical sense. For example much of Isaiah is literally about Cyrus but allegorically about Jesus the Messiah. And so for Abraham, Moses, David, and Jonah–all pre-figure Christ. The church too is pre-figured, all the sacraments, and Mary herself.
So take the return of the Ark to Jerusalem: the ark that contains the signs of the covenant, its extreme holiness and the duty to handle it according to God’s commands, its arduous journey, the three months in the house of Obed-Edom, David leaping before it. All pre-figure Mary, the one who bears the sign of the new covenant, who travels to Elizabeth’s house and stays for three months, where John the Baptist leaps in his mother’s womb. Luke in his Gospel infancy narrative mirrors the chapters from Samuel with an unmistakable intention to tie Jesus to the Davidic prophecy and kingship. It strains logic to believe that the ark that bore the tablets and the staff of the prophet of Exodus, the ark that was so holy a man was smitten dead for his carelessness and disobedience in touching it, could be more holy than the living vessel who by God’s unique favor conceived and carried the son of God and followed him to the cross. She is by God’s favor spotless, untouched, immaculately conceived, sinless always, and a perpetual virgin.
I get the sense that Catholic devotion to this doctrine is more emotional than rational. That doesn’t mean it’s wrong, though.