The devils and those men who die in the state of mortal sin are punished with everlasting sufferings, is an article of Faith (de fide): from the Athanasian Creed.
This Creed, which dates probably from the fifth century, and which within a few generations afterwards received universal recognition by its practically universal use throughout the Church, ends with the words: “those who have done good, will go into life everlasting, but those who have done evil, into eternal fire. – This is the Catholic faith; unless every one believes this faithfully and firmly, he cannot be saved.”
The existence and eternity of the punishments of hell for the reprobate is clearly evidenced from the time of the Prophets. The Prophesy of Isaias (66:24) is outstanding: “And they shall go forth and look on the dead bodies of the men that have rebelled against me; for their worm shall not die, their fire shall not be quenched, and they shall be an abhorrence to all flesh.”
Many interpreters, even among the Protestants, understand this in a direct and particular sense of the destiny of the impious. Confirmation is found in the Book of Daniel (12:2): “And many of those who sleep in the dust of the earth shall awake, some to everlasting life, and some to shame and everlasting contempt.” (In the Hebrew, unto eternal scorn): for, if we look at the context, we see that it is eternal retribution which is being discussed. There is an even more vivid description of this topic in the fourth and fifth chapters of Wisdom.
In the New Testament the dogma is clearly explained: both in regard to reprobation in general and in regard to the eternity of punishments.
As to reprobation or condemnation in general: Christ asserts that he who has sinned against the Holy Spirit is not forgiven unto eternity, and those who give scandal go into Gehenna, into an inextinguishable fire; that the worthless servant is cast into exterior darkness; finally, that those who have not cultivated charity, by definitive judgment descend into eternal fire.
Concerning the eternity of the punishments — the texts are divided into three classes.
First, the punishments of hell are clearly and explicitly called eternal: “They shall suffer the punishment of eternal destruction and exclusion from the presence of the Lord and from the glory of his might…” (2 Thess. 1:9); “Then he will say to those at his left hand, ‘Depart from me, you cursed, into the eternal fire prepared for the devil and his angels…”. These last words have special force from the context, for it is the final judgment, passed by the Supreme Judge, which is being referred to.
Also, it cannot be said that the word, eternal, (In the Greek, aiónios) is oftentimes used in a broad sense to express long duration. In the New Testament this word, which occurs at least seventy times, always designates eternity, properly called, except, perhaps, in two doubtful cases. This is particularly so in regard to the text from Matthew’s Gospel as previously stated, no doubt remains if we consider the context; for therein the torments of the wicked are called eternal in the same sense that rewards of the just are called eternal. The parallelism insists on this interpretation. But according to all, the rewards of the just are to endure without end.
Secondly, the eternity of the punishments is proved from these texts in which the assertion is made that the pains of hell “will have no end”: “ And if your hand or your foot causes you to sin, * cut it off and throw it away; it is better for you to enter life maimed or lame than with two hands or two feet to be thrown into the eternal fire” (Matthew 18:8): “And if your hand causes you to sin, cut it off; it is better for you to enter life maimed than with two hands to go to hell, * to the unquenchable fire” (Mark 9:43).This threat is repeated three times with the greatest emphasis. Similarly, St. Paul speaks very often about the unjust: “Do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither the immoral, nor idolaters, nor adulterers, nor homosexuals, nor thieves, nor the greedy, nor drunkards, nor revilers, nor robbers will inherit the kingdom of God: (1 Cor. 6:9-10). Now the works of the flesh are plain: fornication, impurity, licentiousness, idolatry, sorcery, enmity, strife, jealousy, anger, selfishness, dissension, party spirit, envy, drunkenness, carousing, and the like. I warn you, as I warned you before, that those who do such things shall not inherit the kingdom of God” (Gal. 5:19-21).
Thirdly, the eternity of the punishments is proved from those places in which the “unchangeable” status of the damned as well as the just is spoken of: “but whoever blasphemes against the Holy Spirit never has forgiveness, but is guilty of an eternal sin” (Mark 3:2). Therefore, he will remain guilty eternally and will be punished forever.
Consequently, in many explicit texts and under various forms the eternity of the punishments is described, so that it would have been difficult to explain the eternity more clearly.