Peter Paul Rubens, “The Immaculate Conception”, ca. 1628
Why is it considered such an extraordinary thing that God chose to simply make one person—the Mother of God—the way that all of us would have been, but for the Fall?
In my understanding, the Catholic Church believes that it was not absolutely or intrinsically necessary for the Blessed Virgin Mary to be sinless; only fitting or appropriate (God making the choice).
Thus, even a dogmatic belief in the Immaculate Conception does not in any way impinge upon the Two Natures of Christ. We believe the Immaculate Conception happened, and are required to believe this, but we don't believe that it had to happen exactly as it did (in all possible worlds, etc.), in order for God to be God.
All the Immaculate Conception did was make Mary as Eve was before the Fall (precisely why the Church fathers often called her the Second Eve or New Eve): unfallen and sinless; not subject to original sin. As Cardinal Newman remarked: why is it considered such an extraordinary thing that God chose to simply make one person -- the Mother of God the Son, the Theotokos -- the way that all of us would have been, but for the Fall?
What better person to choose than the one who bore our Lord Jesus in her own body for nine months? It makes perfect sense to me. I never had much difficulty with Marian doctrines (back in my Protestant days) once they were properly explained to me.
The fullest development of the Immaculate Conception, as formulated by Duns Scotus in the 13th century, was careful to state that the Immaculate Conception does not somehow rule out the need for redemption for the Blessed Virgin Mary. She was saved just like the rest of us, and she calls God her Savior in the Magnificat (as St. Thomas Aquinas pointed out).
She was saved by grace even more than the rest of us because by this special act original sin was removed. Mary could hardly play any part in that, seeing that it was at her conception. I doubt that a person at the moment of their conception has a will or any knowledge of theology or soteriology whatever!
Catholics hold that Mary was saved by grace, just like anyone who is saved. God simply performed an extra supernatural act to make her like Eve was before the Fall. God can do anything He wishes, so this is not prima facie impossible, let alone unreasonable, implausible or immediately “unbiblical.” It contradicts nothing in Scripture. Scripture talks about sinless creatures (pre-Fall Adam and Eve, the good angels).
Being sinless, then, is not a biblically incomprehensible position. This is largely my point when I “do” Mariology from Scripture Alone. I know it won't convince most Protestants, but I am trying to show that it is not immediately unbiblical or extra-biblical (as they usually think it is) to hold these beliefs.
The medieval theologians argued that Mary was saved like everyone else (and Jesus died for her, too, as that is where all salvation and redemption comes from), but in a different fashion. They used the analogy of a pit in the forest. If someone falls in and is rescued by a rope from someone up above, they are “saved.” But it is also true that if someone reaches out and “rescues” them before they are about to fall in, they are also “saved.”
The pit represents sin (and original sin), and the rescuer is God and His grace. Mary never fell into the pit, but it doesn't follow that she was not saved or not rescued from it. She certainly was, and it was entirely by God's grace, and must have been, since she had only been in existence for a moment when it happened. But she also had to follow God in her free will. She agreed to be the Theotokos at the Annunciation.
God knew all this in His foreknowledge, but it doesn't mean that Mary had no free will. God simply knew that she would agree and cooperate in and by His grace, and so chose her, knowing that (because He knows all things).
I would say it was “necessary” for Jesus to be born as He was, to show that He was truly a man (the kenosis: Philippians 2:5-9), than for Mary to be sinless in so doing. But we believe that it was altogether fitting for Mary to be sinless, since she bore God the Son. Why does everyone have to be a sinner? Yet that seems to be a key false premise upon which Protestants base their opposition to this glorious doctrine.
The unfallen angels never sinned. It is thinkable for someone to not be a sinner. After all, the Fall, is the abnormality in mankind, not Mary's Immaculate Conception, which merely returned her back to “normal” again. We don't believe that the Immaculate Conception was absolutely necessary, only that it is true, and happened in fact. There is a difference.
Most everything having to do with creatures is contingent or non-necessary. The necessary truths are things like God not being able to sin, or not being able to not exist. That cannot be other than it is, because God is the necessary Being.
Fr. Ryan Erlenbush wrote on The New Theological Movement website (12-8-11):
And this is the great difference between Jesus and Mary: She required a special grace to preserve her from the stain of sin, but our Savior (by virtue of the Incarnation itself, and on account of his divinity) could not possibly have contracted the guilt or debt of original sin in any respect.
While our Blessed Lady, even though Immaculately Conceived, required a Redeemer (namely, her own Son, Christ Jesus), the Lord did not have any need of a Redeemer. No special grace was required (beyond that of the Incarnation itself) to keep the Christ Child from original sin – he could not possibly have contracted it.
In this sense, the dogma of the Immaculate Conception is seen to be fitting and true while (in itself) not absolutely necessary, but the sinlessness of the Lord Jesus is absolutely necessary and super-eminently fitting.