A fellow blogger and convert, Tim Troutman, has made a post describing early Church witnesses to the doctrine of the Immaculate Conception of the Virgin Mary: that she was preserved by God from the stain of original sin from the moment of her conception.
I especially liked this passage from St. Augustine, which I had never read before (with preface):
In a famous and controversial passage of On Nature and Grace, one of the most important treatises that he devoted to the defense of the doctrine of original sin, Augustine had listed the great saints of the Old and New Testaments, who had nevertheless been sinners. Then he continued: “We must make an exception of the holy Virgin Mary, concerning whom I wish to raise no question when it touches the subject of sins, out of honor to the Lord. For from him we know what abundance of grace for overcoming sin in every particular [ad vincendum omni ex parte peccatum] was conferred upon her who had the merit to conceive and bear him who undoubtedly had no sin” pg 191
