There has been a lot of dialog on the CARM boards about Luther's Mariology lately. I have done a lot of work in this area, so I tend to read these threads. Here was a recent Luther quote that was brought up by a Roman Catholic:
Disputation On the Divinity and Humanity of Christ
February 27, 1540
conducted by Dr. Martin Luther, 1483-1546
translated from the Latin textWA 39/2,.92-121 by Christopher B. Brown
On the Immaculate Conception of Mary:
Argument: Every man is corrupted by original sin and has concupiscence. Christ had neither concupiscence nor original sin. Therefore he is not a man.
Response: I make a distinction with regard to the major premise. Every man is corrupted by original sin, with the exception of Christ. Every man who is not a divine Person [personaliter Deus], as is Christ, has concupiscence, but the man Christ has none, because he is a divine Person, and in conception the flesh and blood of Mary were entirely purged, so that nothing of sin remained. Therefore Isaiah says rightly, "There was no guile found in his mouth"; otherwise, every seed except for Mary's was corrupted.
I exhorted this Catholic to stay away from topics he didn't know anything about. A close reading of this quote denies any notion that Mary was purified from sin at her conception. Rather, the focus is on Christ’s conception. Here is Luther’s reasoning:
1. The Holy Spirit was present at Christ’s conception to ensure his sinlessness.
2. During Christ’s conception, the Holy Spirit sanctified Mary so that the child would be born with non-sinful flesh and blood.