Below are some insightful quotes on hell from great theologians. All are found in the book Hell on Trial: The Case for Eternal Punishment, by Robert A. Peterson.
The worshippers of God will be with God for ever, clothed with the proper substance of eternity; but the profane and all who are not wholly devoted to God, in the punishment of the fire which is just as eternal. ~ Tertullian
Now the reason why eternal punishment appears harsh and unjust to human sensibilities, is that in this feeble condition of those sensibilities under their condition of mortality man lacks the sensibility of the highest and purest wisdom, the sense which should enable him to feel the gravity of the wickedness in the first act of disobedience. ~ Augustine
If both are “eternal,” it follows necessarily that either both are to be taken as long-lasting but finite, or both as endless and perpetual. The phrases “eternal punishment” and “eternal life” are parallel and it would be absurd to use them in one and the same sentence to mean: Eternal life will be infinite, while eternal punishment will have an end. Hence, because the eternal life of the saints will be endless, the eternal punishment also, for those condemned to it, will assuredly have no end. ~ Augustine (in reference to Matt. 25:46)
The magnitude of the punishment matched the magnitude of the sin…Now a sin that is against God is infinite; the higher the person against whom it is committed, the graver the sin–it is more criminal to strike a head of state than a private citizen–and God is of infinite greatness. Therefore an infinite punishment is deserved for a sin committed against him. ~ Thomas Aquinas
The fiery oven is ignited merely by the unbearable appearance of God and endures eternally. For the Day of Judgment will not last for a moment only but will stand throughout eternity and will thereafter never come to an end. Constantly the damned will be judged, constantly they will suffer pain, and constantly they will be a fiery oven, that is, they will be tortured within by supreme distress and tribulation. ~ Martin Luther
The perpetual duration of this death is proved from the fact that its opposite is the glory of Christ. This is eternal and has no end. Hence the violent nature of that death will never cease. ~ John Calvin