The "money quote" comes from Pope John Paul II in his 1994 Apostolic Letter "Ordinatio Sacerdotalis":
"Although the teaching that priestly ordination is to be reserved to men alone has been preserved by the constant and universal Tradition of the Church and firmly taught by the Magisterium in its more recent documents, at the present time in some places it is nonetheless considered still open to debate, or the Church's judgment that women are not to be admitted to ordination is considered to have a merely disciplinary force.
"Wherefore, in order that all doubt may be removed regarding a matter of great importance, a matter which pertains to the Church's divine constitution itself, in virtue of my ministry of confirming the brethren (cf. Lk 22:32) I declare that the Church has no authority whatsoever to confer priestly ordination on women and that this judgment is to be definitively held by all the Church's faithful."
This is what is known as an infallible statement by the Pontiff to all of the faithful on a matter of faith. In other words, this is an unalterable and eternal doctrine of the Church.
So, to the Sister Chittister's of the world... enough already!