History and Background
The contemporary statement of the teaching can be found here in the Catechism of the Catholic Church.
Orthodox Church and the doctrine of the Immaculate Conception
Relevant quotes from the Fathers
"...being Himself at once God and man, His flesh and soul were and are holy - and beyond holy. God is holy, just as He was and is and shall be, and the Virgin is immaculate, without spot or stain, and so, too, was that rib which was taken from Adam. However the rest of humanity, even though they are His brothers and kin according to the flesh, yet remained even as they were, of dust, and did not immediately become hold and sons of God."
- - St. Symeon the New Theologian, Discourse XIII in On the Mystical Life, vol. 2, trans. Alexander Golitzin (SVS Press, 1996)