For more resources on this subject, see our earlier article “Catholicism.”
In 1854 at the papal bull Ineffabilis Deus, Pope Pius IX formally defined the doctrine of the immaculate conception. Evangelicals often err in understanding this doctrine. The immaculate conception doesn’t refer to the virgin birth; instead, it refers to the sinlessness of Mary. The Catholic Catechism (1994) explains,
From among the descendants of Eve, God chose the Virgin Mary to be the mother of his Son. “Full of grace,” Mary is “the most excellent fruit of redemption” (SC 103): from the first instant of her conception, she was totally preserved from the stain of original sin and she remained pure from all personal sin throughout her life.
By the grace of God Mary remained free of every personal sin her whole life long.
Finally the Immaculate Virgin, preserved free from all stain of original sin, when the course of her earthly life was finished…
A Critique of the Immaculate Conception
While Mary was called “blessed” (Lk. 1:42) and “favored” (Lk. 1:28), she is no where referred to as sinless. In fact, the Bible states that all human beings are sinful. For instance, Paul argues that “all have sinned and fallen short of the glory of God” (Rom. 3:23). Solomon states, “There is no man who does not sin” (1 Kings 8:46) and “there is not a righteous man on earth who continually does good and who never sins” (Eccl. 7:20). The psalmist writes, “If You, Lord, should mark iniquities, O Lord, who could stand?” (Ps. 130:3) and “For in Your sight no man living is righteous” (Ps. 143:2) and “There is no one who does good” (Ps. 14:1). The Proverbs ask, “Who can say, ‘I have cleansed my heart, I am pure from my sin’?” (Prov. 20:9) Revelation states of God, “You alone are holy” (Rev. 15:4).
When followers tried to venerate Mary in any special way, Jesus rebuked them. For instance, a woman cried out, “Blessed is the womb that bore You, and the breasts at which You nursed” (Lk. 11:27). But Jesus responded, “On the contrary, blessed are those who hear the word of God, and observe it” (v.28). In addition, later in this passage, Mary admits that she needs a Savior. Luke writes, “And my spirit has rejoiced in God my Savior” (Lk. 1:47). If Mary was sinless, then she wouldn’t have needed a Savior.
Catholic apologists sometimes argue that Jesus needed a sinless mother in order for him to be born free from sin. However, if Jesus needed a sinless mother to be born sinless, then why didn’t Mary need a sinless mother to be born sinless? This would create a bizarre regress.
 Catechism of the Catholic Church. Section 508.
 Catechism of the Catholic Church. Section 493.
 Catechism of the Catholic Church. Section 966.