The Immaculate Conception teaches that Mary was preserved free from all stain of original sin from the moment she was conceived. Not that she was a virgin when she bore Jesus. That is a different question. This one says she herself was conceived without sin, and lived without it.
It was made binding dogma by Pius IX on 8 December 1854, in the bull Ineffabilis Deus. Belief in it is required. The Book never says it.
A doctrine that takes eighteen centuries to become mandatory was not apostolic.
Duns Scotus, in the 13th century, argued that God could have preserved Mary from sin, and that it was fitting for the mother of Jesus. Fitting is not revealed. The Franciscans took it up. Others disputed it for centuries.
Sixtus IV approved a feast day. That did not settle the question. Pius IX, under pressure from clergy and the devout, closed it by papal authority in 1854 and called the result revealed by God.
By the 19th century, devotion to Mary was thick in Catholic life. A sinless mother felt right to the people who already prayed to her. The doctrine followed the devotion. It did not lead it.
Catholic theologians argued over it for centuries. If it had been essential, it would have been plain from the start. A pope declaring it does not make a late opinion into Scripture.
All. The Book makes no exception for Mary. The only one without sin is Jesus. "For we have not an high priest which cannot be touched with the feeling of our infirmities; but was in all points tempted like as we are, yet without sin." Hebrews 4:15.
Mary said that. A woman preserved from all stain of original sin, and living without sin, does not need a Saviour. She named God as hers. That one sentence ends the dogma.
Sin entered by one man. Death passed upon all men. Conception in sin is the ordinary state of those born of Adam. The Book never lifts Mary out of that. It lifts Jesus out of it, and it tells us how.
This assumes sin is passed through the mother, and that a sinful mother would have stained the child. The Book says sin entered by Adam, and that the holiness of the child came another way.
Therefore. The holy thing is holy because the Holy Ghost came upon her and the power of the Highest overshadowed her. Not because she was already sinless. The verse explains the child's holiness. It does not require the mother's.
God's words say "highly favoured." Luke 1:28. That is a greeting. God had chosen her for a work. Catholic Bibles, following the Latin, say "full of grace." The Douay-Rheims has "Hail, full of grace." From that phrase a sinless storehouse of grace is built.
The Greek is a verb. She was favoured by God. It does not say she was conceived without sin, and it does not say she never sinned. "Full of grace" is the rendering that props the dogma up. "Highly favoured" does not.
A pope cannot add to God's words and call the addition revealed. "If any man shall add unto these things, God shall add unto him the plagues that are written in this book." Revelation 22:18. Vague praise of Mary's purity in later writers is not a doctrine of her conception. Poetic honour is not dogma. The clear teaching takes precedence over a tradition that spent centuries undecided.
Mary was highly favoured. She bore the Christ. She believed, and she rejoiced in God her Saviour. Only Jesus is without sin. He died for our sins according to the scriptures, was buried, and rose again the third day according to the scriptures. That includes her. It includes us.