It may depend on who you’re talking to, but for me, it was Mary;
Mary as the Ark of the New Covenant, which means she HAD to be sinless, and without original sin, etc, which I had never even heard before in all my years as a Baptist.
Mary as Mother of all Christians, seeing as she could not just be mother of Jesus, as head of the Church, but must also be mother of the body, the Christians who make up the Church.
And, on side notes, the contradictory notion of sola scriptura, and through that to the idea of authority and apostolic succession (see Acts 1), the historical truth of the Catholic Church being the first Church, the ECF’s on the Eucharist, the need for faith AND works, and so through that the need for confession, baptism (not believer’s baptism), etc.
It mostly depends on the person’s willingness to understand and accept the ideas as they are. Would probably be different for others.