Human beings have inherited the burden of the first sin against God, which is dramatized in the story of Adam and Eve in the Book of Genesis. We are cleansed of that burden by the sacrament of baptism, through which we receive the grace Jesus won for us with his death, resurrection, and ascension into heaven.
Mary, who was to be the mother of God, was spared the stain of that sin.
A feast based on this mystery was celebrated in Syria as early as the fifth century. The doctrine of the Immaculate Conception was proclaimed by Pope Alexander VII in 1661, and it was defined as a dogmatic teaching of the church by Pope Pius IX in 1854.
Adapted from RENEW International's At Prayer With Mary, available in our online bookstore.