No one who knowingly and deliberately rejects the truth will be saved. It doesn’t matter how good of a Muslim, Jew, Baptist, or anything else he may be. If anyone rejects the truth of Christ and his Church—even one definitive teaching—they will be lost. - Catholic Answers
This is summary point #1 in a Roman Catholic article explaining extra ecclesiam nulla salus which is an infallible statement meaning 'outside the Church there is no salvation'. The article talks, in part, about 'invincible ignorance' regarding not just commission of material sins but also of salvific truth:
“Those who, through no fault of their own, do not know the Gospel of Christ or his Church, but who nevertheless seek God with a sincere heart, and, moved by grace, try in their actions to do his will as they know it through the dictates of their conscience – those too may achieve eternal salvation” (quoting, Lumen Gentium, 16).
*Note: The article uses the terms 'Catholic Church' and 'Catholic Faith' frequently and reading it one assumes 'Roman Catholic' is intended rather than the older definition of catholic as 'universal'. Likewise, when 'Church' is used the assumption is that Roman Catholic Church is intended. If I am wrong in this assumption and Roman Catholic Church is not intended, please correct me in comments as it will drastically change and perhaps even invalidate this question.
The article lays out that anyone who is knowledgeable of and in rejection of "even one definitive teaching" of Christ's Church will be lost. They will not be saved unless they repent and accept that teaching that they have previously rejected.
I have always understood the 4 Marian Dogmas to be necessarily believed in order to be a member in good standing of the Roman Catholic Church:
Through the authority of His Church on earth, God has revealed to us four essential truths about Mary’s identity, otherwise known as “the Marian dogmas.” To be in full communion with Christ—the Church reminds us—we must believe in each of these dogmas. In other words, they aren’t optional beliefs. - Good Catholic
But now, knowing that these Marian Dogmas are definitive, infallible teachings of the RCC and putting them in combination with the first quote (above), it appears that anyone who knowingly and deliberately rejects even one Marian Dogma will be lost. In other words, any member of any Protestant denomination or Church whatsoever who has what the RCC considers to be valid baptism and marriage sacraments are still completely and hopelessly lost if they knowingly and deliberately reject say, the 'Perpetual Virginity of Mary' for example.
A Roman Catholic answer to a a previous question clearly stated that the perpetual virginity of Mary bestows no salvific benefit:
Even without the perpetual virginity of Mary, our salvation in Christ not lessened! Christ would still be Our Redeemer and Mary would still be the Mother of Jesus, the Church founded by Christ, and all mankind. However, it would be seen through the eyes of the Church in a totally different perspective!
Perhaps for a Roman Catholic the perpetual virginity of Mary adds a depth of understanding to the entire plan of salvation, the nature of God, etc., and that is fine, but it seems a contradiction to declare that a person will be lost unless they believe a doctrine that has no salvific benefit.
My question is: If salvation in Christ is not lessened without the perpetual virginity of Mary why must one believe the Dogma or be lost?