CathInfo.com

Rahner in Theological Investigations:

This optimism concerning salvation [of non-Catholics] appears to me one of the most noteworthy results of the Second Vatican Council. For when we consider the officially received theology concerning all 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.

Rahner was right on the money, and I had been making this same observation for a long time before I found this quote.  Of course, he refers to theology being "more or less traditional right down to the Second Vatican Council."  In the years preceding the Council, it was mostly "less" traditional, and this explains why, to Rahner's amazement, "how little opposition the conservative wing of the Council [among the leaders of which was Archbishop Lefebvre] brought to bear on this point."  Rahner points out how this flew under the radar and didn't even create a stir among the conservatives.  That tells us something.  Father Feeney was right.

We have the undeniably thrice-defined (and oft reaffirmed) dogma of EENS.  There's only one way to increase this "optimism concerning salvation".

MAJOR:  There's no salvation outside the Church. [dogma]
MINOR:  Heretics, schismatics, and even infidels can be saved.
CONCLUSION:  Church can include heretics, schismatics, and even infidels.

In order to extend salvation to non-Catholics, since no one can deny the dogma EENS, one must extend the Church to be inclusive of non-Catholics also.

This is in fact none other than the subsistence ecclesiology of Vatican II.

Then, by extension, how can these non-Catholics be really Catholics?  By subjectivizing faith, and reducing faith to good will, sincerity, intention to do God's will, etc.

Religious Liberty then follows logically from this.

MAJOR:  Men have a right to please God and to save their souls.
MINOR:  Men please God and save their souls by following their consciences, even if they are erroneous.
CONCLUSION:  Men have a right to follow their consciences, even if erroneous.

If men save their souls even by following their false consciences and through false religions, then to hinder and to prevent them from doing so means hindering their salvation and their ability to please God.

The proof is that Karl Rahner was undoubtedly, unabashedly and proudly a Modernist.  If he had found even one Church Father who agreed with his V2-mindset, he would've exploited that quote to the nth degree.  It would've been repeated by every Modernist til they were blue in the face.  But they never found anything...

That would sorta undermine the principle of modernism to go with a direct quote tho