2 added Cantate Domino quote
source | link

The first sentence of the Athasasian Creed says:

Whoever wishes to be saved, needs above all to hold the Catholic faith; unless each one preserves this whole and inviolate, he will without a doubt perish in eternity.

Heretics do not preserve "the Cathloic faith…whole and inviolate"; thus, they "will without a doubt perish in eternity."

There's also Pope Eugene IV's Cantate Domino:

[The Catholic Church] firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart “into everlasting fire which was prepared for the devil and his angels” [Matt. 25:41], unless before the end of life the same have been added to the flock

A heretic either "is unwilling to assent to Christ" or "chooses not what Christ really taught, but the suggestions of his own mind"; "heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas" (source).
(cf. Pope Leo XIII's Satis Cognitum on the necessity of holding the whole Catholic faith)

The first sentence of the Athasasian Creed says:

Whoever wishes to be saved, needs above all to hold the Catholic faith; unless each one preserves this whole and inviolate, he will without a doubt perish in eternity.

Heretics do not preserve "the Cathloic faith…whole and inviolate"; thus, they "will without a doubt perish in eternity."

A heretic either "is unwilling to assent to Christ" or "chooses not what Christ really taught, but the suggestions of his own mind"; "heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas" (source).
(cf. Pope Leo XIII's Satis Cognitum on the necessity of holding the whole Catholic faith)

The first sentence of the Athasasian Creed says:

Whoever wishes to be saved, needs above all to hold the Catholic faith; unless each one preserves this whole and inviolate, he will without a doubt perish in eternity.

Heretics do not preserve "the Cathloic faith…whole and inviolate"; thus, they "will without a doubt perish in eternity."

There's also Pope Eugene IV's Cantate Domino:

[The Catholic Church] firmly believes, professes, and proclaims that those not living within the Catholic Church, not only pagans, but also Jews and heretics and schismatics cannot become participants in eternal life, but will depart “into everlasting fire which was prepared for the devil and his angels” [Matt. 25:41], unless before the end of life the same have been added to the flock

A heretic either "is unwilling to assent to Christ" or "chooses not what Christ really taught, but the suggestions of his own mind"; "heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas" (source).
(cf. Pope Leo XIII's Satis Cognitum on the necessity of holding the whole Catholic faith)

1
source | link

The first sentence of the Athasasian Creed says:

Whoever wishes to be saved, needs above all to hold the Catholic faith; unless each one preserves this whole and inviolate, he will without a doubt perish in eternity.

Heretics do not preserve "the Cathloic faith…whole and inviolate"; thus, they "will without a doubt perish in eternity."

A heretic either "is unwilling to assent to Christ" or "chooses not what Christ really taught, but the suggestions of his own mind"; "heresy is a species of unbelief, belonging to those who profess the Christian faith, but corrupt its dogmas" (source).
(cf. Pope Leo XIII's Satis Cognitum on the necessity of holding the whole Catholic faith)