"The doctrine which holds that the Blessed Virgin Mary, from the first instant of her conception, was, by a most singular grace and privilege of Almighty God, in view of the merits of Jesus Christ, the Redeemer of the human race, preserved from all stain of Original Sin, is a doctrine revealed by God, and therefore to be firmly and steadfastly believed by all the faithful" (1911 Encyclopedia Britannica section on the Immaculate Conception)
In the vast majority of cases, Catholic dogmas are declared out of ecumenical councils that are convened (e.g. the Council of Trent from 1545 - 1563). In the case of Mary's believed immaculate conception, however, the Pope only sought input from bishops. He ultimately, without convening a council, declared this belief dogmatic based strictly on his authority (known as "ex cathedra") as the head of the church. Pope Pius XII, in 1950, used this authority to declare the belief in her bodily assumption into heaven, at the end of her life, as dogmatic for Catholics.
The church claims that Mary's purity while being conceived is hinted at by several "church fathers" such as Origen (c. 184 - c. 253 A.D.), Augustine (354 - 430), Tertullian (c. 155 - c. 240) and others. It is known that the festival of the Conception of the Blessed Virgin was celebrated in the Greek Church in the 7th century A.D. The explicit question, however, concerning Mary being conceived immaculate (in a state of perfection) seems to have been first raised in the 11th century by St. Bernard of Menthon (from whom the St. Bernard dog is named).
This doctrine is part of a much broader package of Roman Catholic beliefs regarding Mary (the theological study of whom is known as Mariology). They include the teachings that she was a perpetual virgin (she never had sex and did not have children other than Jesus), she lived a sinless life (a belief Martin Luther supported), that she is the mother of God, plus others.
Is it Biblical?
Officially, the Catholics make an eye-opening admission regarding this tenet of their faith. In the 1913 Catholic Encyclopedia, in an article dedicated to explaining the immaculate conception, under the subheading "proof from Scripture," it states the following.
"No direct or categorical and stringent proof of the dogma can be brought forward from Scripture."
Most bishops agreed with Pope Pius IX regarding elevating this belief to the status of a dogma. A few, however, did not, such as the Archbishop of Paris (Sibour). He stated the doctrine was unnecessary and "could be proved neither from the Scriptures nor from tradition, and to which reason and science raised insolvable, or at least inextricable, difficulties" (The Creeds of Christendom, chapter 4, Papal Definition of the Immaculate Conception).
Thomas Aquinas (1225 - 1274), a Catholic priest who is considered one of the greatest medieval theologians, REJECTED this teaching before it ever became dogma (Summa Theologia, part iii., quaest. 27, art. 3). Part of his argument was that she had to be a sinner (see Romans 3:10, 23, Galatians 3:22, 1John 1:8, etc.) in order for her to need a Savior (see Luke 1:47). Bonaventure, an influential Catholic theologian who was a contemporary of Aquinas (1221 - 1274), hesitated to accept this teaching for a similar reason (1911 Encyclopedia Britannica, page 334).
The doctrine that Mary was conceived "immaculate" was not taught by Jesus Christ, his apostles, the apostle Paul, or any true Christian in the first century A.D. As stated by the Catholics (and MANY others), this teaching is entirely void of any Biblical support. Its promulgation rests entirely on raw assertions made by men who believe they have the authority to dictate the faith of others. Since the Bible is to be man's sole foundation on which to determine doctrine (2Timothy 3:16 - 17), this belief deserves to be completely rejected.