Alternate expressions of the proposition:
1. One who knowingly, consciously, and willingly publicly asserts a proposition that is in direct contradiction to a Church teaching that must be believed with Divine and Catholic Faith separates himself from the Church by that very fact.
2. One who knowingly, consciously, and willingly publicly denies or doubts a Church teaching that must be believed with Divine and Catholic Faith separates himself from the Church by that very fact.
Clarifications:
1. One who is separated from the Church is not a member of the Church. He goes from being Catholic to non-Catholic.
2. The separation takes place at the very instant the sin is committed.
3. The proposition is independent of its application to a particular case.
First, you should differentiate between the sin of formal heresy, and the possible sin material heresy:
Fr. Hesse:
Objective (or Material) heresy is: "According to the Church, salvation is attainable outside the Church".
Formal (or Manifest) heresy is: "I don't care what the Church teaches, the Church is wrong, I say salvation is attainable outside the Church."
Second, it might help to differentiate between the heresy of (A) a Catholic, and (B) one who never was Catholic so has always been separated from the Church. Of course for this thread, (B) is pretty much irrelevant.
We know that a penitent heretic (A) can receive absolution of his sin of heresy through confession, or in danger of death can receive all of the sacraments of the Last Rites (Confession, Communion and Extreme Unction) and thereby the censure is removed and he is absolved of his sin of heresy (and all of his other sins) by any priest, whereas (B) those separated from the Church are forbidden by the Church to receive the sacraments.
How do you explain that one who is presumably no longer a member of the Church (A) can receive the sacraments at all.
Also, how is what you say in your quote above for the sin of heresy is not also true for, say, the sin of Adultery?