I’m careful.
“It will be readily understood that in some circumstances, when the question is put explicitly for the first time, the faithful have hesitated. It is even natural that the theologians should show more hesitation than the other faithful. More aware of the apparent opposition between the new opinion and the ancient truth, they may legitimately resist, while awaiting fuller light, what may seem to them unreflecting haste or unenlightened piety.”
This is the seed argument...it was there as a seed, unseen or barely noticed, but grew into a tree that made it so obvious it became dogma - “an extraordinary infallible statement published by a pope or an ecumenical council concerning a matter of faith or morals, the belief in which the Catholic Church requires of all Christians” (Wiki)
According to Catholic teaching, I’m required, if Christian, to accept something that was not taught by the Apostles. “Anyone who runs ahead and does not continue in the teaching of Christ does not have God” - so, how far ahead do you have to get before you cease to be a true church?
Yes, that's the mental block isn't it? That this is some form of novelty. You will find nothing in Scripture or in any of the apostolic writings which contradicts the dogma of the Immaculate Conception and you will find significant evidence to support it.
Firstly, the angel's salutation to Mary recorded in Luke's Gospel as "full of grace" is one such example, the term "full of grace" being understood to mean a unique, supernatural state of soul. The writings of Justin, Irenaeus, Cyril of Jerusalem among others of the early Church fathers all developed this theme and it has been addressed by many other saints of the Church down through the centuries.
“Anyone who runs ahead and does not continue in the teaching of Christ does not have God” - so, how far ahead do you have to get before you cease to be a true church?
The Catholic Church being built on the Rock, Peter and being the recipient of those promises given to Peter will never cease to be a true Church for it is the Mystical Body of Christ.
As always, these discussions inevitably founder on the issue of authority and the promises Christ made to His Church to keep it free from error.
Those promises do not apply to individuals of course, hence the quote you supplied refers to "anyone". Individuals have and continue to apostatize. The authority of the Church gives it the power to "bind and loose" as Christ promised Peter.
As for "how far ahead do you have to get before you cease to be a true Church", well there is only one true Church. "There is one God, one faith, one baptism" (Ephesians).
Ergo all Churches bar one are in error to some degree.