Happy Feast Day: Immaculate Conception of Our Blessed Lady

Our Blessed Lady in the womb of St Anne

I found this information helpful at New Advent...

'No controversy arose over the Immaculate Conception on the European continent before the twelfth century [...]

In the thirteenth century the opposition was largely due to a want of clear insight into the subject in dispute. The word "conception" was used in different senses, which had not been separated by careful definition. If St. Thomas, St. Bonaventure, and other theologians had known the doctrine in the sense of the definition of 1854, they would have been its strongest defenders instead of being its opponents.

We may formulate the question discussed by them in two propositions, both of which are against the sense of the dogma of 1854:

the sanctification of Mary took place before the infusion of the soul into the flesh, so that the immunity of the soul was a consequence of the sanctification of the flesh and there was no liability on the part of the soul to contract original sin. This would approach the opinion of the Damascene concerning the holiness of the active conception.

The sanctification took place after the infusion of the soul by redemption from the servitude of sin, into which the soul had been drawn by its union with the unsanctified flesh. This form of the thesis excluded an immaculate conception.

The theologians forgot that between sanctification before infusion, and sanctification after infusion, there was a medium: sanctification of the soul at the moment of its infusion. To them the idea seemed strange that what was subsequent in the order of nature could be simultaneous in point of time. Speculatively taken, the soul must be created before it can be infused and sanctified but in reality, the soul is created and sanctified at the very moment of its infusion into the body. Their principal difficulty was the declaration of St. Paul (Romans 5:12) that all men have sinned in Adam. 

The purpose of this Pauline declaration, however, is to insist on the need which all men have of redemption by Christ. Our Lady was no exception to this rule. A second difficulty was the silence of the earlier Fathers. But the divines of those times were distinguished not so much for their knowledge of the Fathers or of history, as for their exercise of the power of reasoning. They read the Western Fathers more than those of the Eastern Church, who exhibit in far greater completeness the tradition of the Immaculate Conception. And many works of the Fathers which had then been lost sight of have since been brought to light...

...for more click here. Read Ineffabilis Deus (God Ineffable), the Papal Bull of Pope Pius IX on the Immaculate Conception of Our Lady here.