Karl Rahner on Vatican II:
It was declared at the Second Vatican Council that atheists too are not excluded from this possibility of salvation… The only necessary condition which is recognized here is the necessity of faithfulness and obedience to the individual’s own personal conscience. This optimism concerning salvation appears to me one of the most noteworthy results of the Second Vatican Council. For when we consider the officially received theology concerning these questions, which was more or less traditional right down to the Second Vatican Council, we can only wonder how few controversies arose during the Council with regard to these assertions of optimism concerning salvation, and wonder too at how little opposition the conservative wing of the Council brought to bear on this point, how all this took place without any setting of the stage or any great stir even though this doctrine marked a far more decisive phase in the development of the Church’s conscious awareness of her Faith than, for instance, the doctrine of collegiality in the Church, the relationship between scripture and tradition, the acceptance of the new exegesis, etc.
By V2, even the "conservative" Fathers had been poisoned, as we read from +Lefebvre's words about salvation, which are almost identical to Rahner's "Anonymous Christian" position.
We find this continuing today among Trad Catholics, where 95%+ of Trad clergy promote the notion that non-Catholics can be saved.
And yet this error is at the heart of Vatican II, as Rahner (who should know, since he was highly influential there) notes here.
MAJOR: There's no salvation outside the Church. [Dogma]
MINOR: Non-Catholics can be saved. [position held by 95%+ of Trad clergy]
CONCLUSION: Non-Catholics can be within the Church.
Here you have Vatican II ecclesiology in a nutshell. Since no one can deny the thrice-defined dogma that there's no salvation outside the Church, the only way to get non-Catholics saved is to claim that they can be in the Church somehow. What does that do to the definition of Church, to ecclesiology?
So, while the Catholic Church constitutes the visible "subsistent" core of the Church, the Church also includes various non-Catholics. These non-Catholics must be formally in the Church, even if they are not fully united materially (visibly) to the Church, and they have varying degrees of separation from the Catholic Church. They are in a true sense our separated brethren. Ecuмenism then entails a recognition of this unity, while continuing to dialog about our material / accidental differences.
Religious Liberty also follows. If people please God and save their souls by following their (even erroneous) consciences, then since people have a right to please God and save their souls, they have a right to follow their (even erroneous) consciences. In fact, to thwart them from following their (even erroneous) consciences would be to imperil their salvation.
And yet we have Trads continuing to adamantly claim that non-Catholics can be saved ... while denouncing Vatican II ecclesiology as heretical, while ironically holding to the same ecclesiology themselves.