Mary and her family were Jews, so they would have believed that all people enter the world free of sin with an innocent and pure soul that’s untainted by sin. Sin enters the world for each individual and tarnishes their souls only on occasion when they choose to commit personal sins. So, I suppose they believed that everyone is immaculately conceived, but according to their notion of sin. The Christian concept of original sin holds no place in Judaism, though there were a few rabbis in Talmudic times who taught that death is a punishment for Adam’s sin, which reminds us what Paul has to say in his Epistle to the Romans 5:12. I believe the idea of original sin as it has evolved in Christianity originated with Paul and not with any of the Twelve, including Peter. But our chief shepherd would have had to approve of Paul’s idea before he could annunciate it to the churches. Many Protestants mistakenly believe that all the essential Christian doctrines were definitively formulated by the Apostles and their appointed associates by the time of their deaths and preserved in sacred Scripture. And so, they refuse to believe in Mary’s Immaculate Conception and Assumption because these events “aren’t in the Bible” - at least not explicitly, is the Catholic rejoinder.
What the apostles themselves comprehensively thought about Mary’s standing with God, we can never know. But I’m sure they owned they hadn’t yet grasped all the implications and its full significance. And what they did understand was that Jesus had sent the Paraclete to guide the Church in “all” truth. In other words, the Spirit would help the Church to develop in knowledge of the divine mysteries over a span of time (John 16:12-13). For this reason, our Lord founded his Church through which the wisdom of God in all its mystery can be better known and more fully understood as time goes on. (Ephesians 3:10-11). In his First Letter to Timothy, Paul affirms what he means to say to the Ephesians: “the Church is the pillar and foundation of the truth” (3:15). And by “Church” he meant that which was instituted at his time: one, visible, hierarchical corporate body, which we don’t have in Protestantism with all its countless, autonomous denominational churches with no one Spirit guiding them since apostolic time. A great chasm of time separates the Protestant churches from the day Christ’s one apostolic Church was born on Pentecost Sunday. And the Bible, which was compiled three centuries after the Church’s birth, having proceeded from sacred Tradition, evidently serves as a poor bridge to cross that great divide. Jesus ensured that no bridge should ever have to be constructed so that the Church’s unity of faith would be preserved through all future generations of the faithful. And he did this by having the apostles appoint able men who would succeed them in their divine office and carry on their ministry after they are gone. Paul writes to Timothy, who has been appointed bishop: “What you have heard from me before many witnesses entrust to faithful men who will be able to teach others also” (2, 2:2). These commissioned men Paul speaks of have been ordained by the apostles and sealed with the Holy Spirit as a guaranty to teach the truth that has been handed down to them and placed in their custody. (2 Corinthians 1:21-22). Indeed, the apostle calls his ministry an “office” which presupposes there are successors to it (Colossians 1:25).
The Bible cannot serve as the final rule of faith, because not all the material in it is presented in an explicit and definitive way. The Bible is materially sufficient, but formally insufficient. For us to know what God has revealed, we must turn to the Church which Christ founded on Peter and the Apostles. And we must turn our gaze upon the sacred Tradition of the Church which belongs to the deposit of faith together with sacred Scripture. God’s unwritten word - that which is declared by the Holy Spirit to the Church - clarifies what is meant in God’s written word that has followed. Protestants deny the Real Presence of Christ in the Holy Eucharist by transubstantiation because they don’t see this in the NT. And because it isn’t explicitly there, the dogma must be false. However, if we look to Tradition, we’ll see that it is there and has always belonged to the deposit of faith from the beginning, and taught by the divinely constituted teaching office of the Church. Ignatius, Bishop of Antioch, was a disciple of the apostle John (See Chapter 6 of his gospel.), and this is what he has written c .A.D. 110: “They abstain from the Eucharist and from prayer, because they confess not the Eucharist to be the flesh of our Saviour Jesus Christ, which suffered for our sins, and which the Father, of His goodness, raised up again” (Epistle to Smyrnaeans, 7,1). The word “Eucharist” isn’t in the Bible, but it’s there in Tradition. We can say the same thing about Mary’s Immaculate Conception. The word isn’t written down in Scripture, but this divine truth has been declared by the Holy Spirit to the Church: “Thou alone and thy Mother are in all things fair, there is no flaw in thee and no stain in thy Mother.” [Ephraem, Nisibene Hymns, 27:8 (A.D. 370)] Thus, we mustn’t ask ourselves what Mary or the apostles might have known or fully understood after Christ’s ascension into heaven, or even after Pentecost, because we can never know. All we can know is what the Church has gradually understood and taught in the course of time under the guidance of the Holy Spirit, as she matures in the knowledge of God and His mighty deeds.