Dear brother Marc,
Does anyone have any sayings from the fathers on this topic? Just so we can all agree, let's limit these saying to those pre 5th century.
I don't think there are any explicit sayings. I think the belief came out of a pious meditation on such statements about Mary being the New Eve (extant since the second century), combined with the Church's belief on her being the most perfect creation of God (also just as ancient). It finally came to fruition, as mentioned, with the establishment of the Feast of the Conception in the Byzantine Church in the 7th or 8th century. Of course, the Feast is not exactly about her preservation from the stain of original sin - it is, rather, about the pious belief that Mary received the graces of the Holy Spirit at her conception. I'm guessing that the belief quickly blossomed into the realization that receiving all the graces of the Holy Spirit (that a creature can receive, that is) is tantamount to receiving Baptism. So what is the effect of Baptism? The cleansing of the stain of original sin.
When the Feast migrated to the West, it faced opposition. What the Latins already
believed was that Mary received the graces of the Holy Spirit at the moment of her SPIRITUAL conception (i.e., ensoulment or quickening). However, unlike the Easterns, the Latins distinguished the moment of ensoulment from the moment of physical conception, believing that ensoulment occurred at least 40 days or more after
the physical conception. Thus, some prominent Saints in the Latin Church opposed the introduction of the Feast in Latin Church. They could not agree to the idea that she received the graces of the Holy Spirit from the moment of her conception, because they believed she received those graces at least 40 days hence. As part of their rhetoric against the Eastern Feast, they opined that only Jesus was absolutely pure from the moment of his physical conception because he did not have a human father (from whom original sin was traditionally held to be transmitted).
Eventually, the Latin Church grew to understand, together with the East, that the moment of ensoulment occurs at the same instant as the moment of conception. Hence, the dogma of the IC.
The above explanation demonstrates that there is no difference between what the medieval Latin Fathers (who opposed the introduction of the Feast into the West) believed and what Catholics today believe - namely, that Mary received all the graces the Holy Spirit at the moment of her ensoulment. The dogma of the IC uses the word "conception" instead of "ensoulment" simply because both occur at the same time (it's the simple and basic commutative law).
There are some things that need to be emphasized about the dogma of the IC that opponents always mispresent (and those misrepresentations are glaringly evident in this thread):
1) The dogma does NOT say that Mary was preserved from original sin. If she was preserved from original sin PERIOD, then she would not only be spiritually pure, but she would also not experience corruption or death. But that is, as stated, NOT what the dogma states (contrary to the polemical wishes of Father Ambrose). Rather it says she was preserved from the STAIN of original sin. As repeatedly explained, the STAIN of original sin refers to the SPIRITUAL CONSEQUENCES of original sin, NOT the PHYSICAL/TACTILE effects of original sin.
2) The dogma, when it speaks of conception, refers to her SPIRITUAL conception (i.e., ensoulment), not her PHYSICAL conception. Thus, it is true, and the dogma does not contradict, the teaching of the medieval Latin Fathers that only Jesus had an immaculate PHYSICAL conception.
3) The two points emphasized above evinces that the dogma does not contradict the fact that Mary died.
4) The dogma of the IC refers to nothing more nor less than the fact that Mary received the graces of Baptism at the moment of her conception, as already explained fully in an earlier post.