The Chatholic doctrines are defined formally only when there is a controversy that needs to be cleared up or when the magisterium (the Church in its office as teacher; cf. Matt. 28:18–20; 1 Tim. 3:15, 4:11) thinks the faithful can be helped by particular emphasis being drawn to some already-existing belief.
The definition of the Immaculate Conception was prompted by the latter motive; it did not come about because there were widespread doubts about the doctrine. In fact, the Vatican was deluged with requests from people desiring the doctrine to be officially proclaimed.
Pope Pius IX, who was highly devoted to the Blessed Virgin, hoped the definition would inspire others in their devotion to her.
Complement to the dogma of the Immaculate Conception, over the centuries, as I posted some things the early chruch Fathers and the Doctors of the Church spoke often about the fittingness of the privilege of Mary’s Assumption. The speculative grounds considered include Mary’s freedom from sin, her Motherhood of God, her perpetual virginity, and the key union with her and the salvific work of Christ.
The dogma is especially fitting when one examines the honor that was given to the ark of the covenant. It contained the manna (bread from heaven), stone tablets of the ten commandments (the word of God), and the staff of Aaron (a symbol of Israel’s high priesthood). Because of its contents, it was made of incorruptible wood, and Psalm 132:8 said, "Arise, O Lord, and go to thy resting place, thou and the ark of thy might."
If this vessel was given such honor, how much more should Mary be kept from corruption, since she is the new ark—who carried the real bread from heaven, the Word of God, and the high priest of the New Covenant, Jesus Christ.
Some argue that the new ark is not Mary, but the body of Jesus. Even if this were the case, it is worth noting that 1 Chronicles 15:14 records that the persons who bore the ark were to be sanctified. There would be no sense in sanctifying men who carried a box, and not sanctifying the womb who carried God himself!
After all, wisdom will not dwell "in a body under debt of sin" (Wis. 1:4 NAB).
But there is more than just fittingness. After all, if Mary is immaculately conceived, then it would follow that she would not suffer the corruption in the grave, which is a consequence of sin [Gen. 3:17, 19].
Mary freely and actively cooperated in a unique way with God’s plan of salvation (Luke 1:38; Gal. 4:4). Like any mother, she was never separated from the suffering of her Son (Luke 2:35), and Scripture promises that those who share in the sufferings of Christ will share in his glory (Rom. 8:17). Since she suffered a unique interior martyrdom, it is appropriate that Jesus would honor her with a unique glory.
All Christians believe that one day we will all be raised in a glorious form and then caught up and rendered immaculate to be with Jesus forever (1 Thess. 4:17; Rev. 21:27). As the first person to say "yes" to the good news of Jesus (Luke 1:38), Mary is in a sense the prototypical Christian, and received early the blessings we will all one day be given.
Since the Immaculate Conception and Assumption are not explicit in Scripture, Fundamentalists conclude that the doctrines are false.
Here, of course, we get into an entirely separate matter, the question of sola scriptura, or the Protestant "Bible only" theory.
There is no room in this tract to consider that idea. Let it just be said that if the position of the Catholic Church is true, then the notion of sola scriptura is false. There is then no problem with the Church officially defining a doctrine which is not explicitly in Scripture, so long as it is not in contradiction to Scripture.
The Catholic Church was commissioned by Christ to teach all nations and to teach them infallibly—guided, as he promised, by the Holy Spirit until the end of the world (John 14:26, 16:13). The mere fact that the Church teaches that something is definitely true is a guarantee that it is true (cf. Matt. 28:18-20, Luke 10:16, 1 Tim. 3:15).