When we interpret “forever” in English, as well as in Greek, context is king. For example, when someone gets back from the DMV and says to us, “that took forever,” what do we interpret that phrase to mean? We take a number of things into account in our interpretation. We understand that it takes a relatively long time at the DMV and we understand that people very often joke about how long it takes at the DMV. We also take into account that the person is standing in front of us saying, “that took forever” which clearly demonstrates that it did not in fact literally take forever.
The person that said “forever” was using it as an expression for “a long time.” However, if that same person said, “God is forever” we would understand that we need to interpret that “forever” differently. Why? Because context is king. And context is telling us that the referent in this case is “God,” not the DMV, and that fact changes the meaning of the word “forever.”
The Bible tells us that certain things are eternal/everlasting. For instance, God is eternal (Rev. 4:9-10), Jesus is alive forevermore (Rev. 1:18), heaven is eternal (Jn. 3:16), and judgment in hell is eternal. If we say judgment in hell is not eternal then we lose grounds for saying that God, Jesus, and heaven are eternal since the same words are explicitly and very intentionally used to express the eternity of each subject under question (and the eternality of hell and heaven are even paralleled in Matthew 25:46).
So, it seems like it is clear that hell will last as long as God does and that hell will last as long as heaven does. However, we do not just have to look at the word “eternal” or “forever.” We can also look at other words and coupling of words to get the intended meaning. Revelation 14:11 says “the smoke of their torment goes up forever and ever, and they have no rest, day or night.” Even if you translate this verse “the smoke of their torment goes up ages of ages and they have not rest, day or night” it is still explicit that the judgment has no end. And it certainly does not appear that there will finally be universal salvation.
There are also many other texts to consider. For instance, Mark 9:48 says the “worm does not die and the fire is not quenched” (cf. Is. 66:24). Revelation 14:9-11 and 20:10 says “forever and ever.” The fact that the forms used are plural in number further reinforces the idea of never-ending duration. Matthew 3:12 talks about “unquenchable fire.”
The emphatic construction that is used to communicate to us that punishment is “forever and ever” is found in other places as well. In Revelation, we see places where the construction refers to the unending worship of God (see 1:6; 4:9; 5:3-13), the endless life of God (4:10; 10:6), and the everlasting reign of the saints (22:5). The language used to describe the duration of hell is unambiguous, emphatic, and conclusive: Punishment in hell is eternal.
This is not something I relish. I do not write this because I could care less about people. Actually, my motivation is that I do care for people. Because if what the Bible says is true—that God is a just and holy God that will punishment people unless they turn in repentance to trust Jesus for salvation—then this is very serious and worthy of attention.
 Alan W. Gomes, “Evangelicals and the Annihilation of Hell, Part One,” in Christian Research Journal, Spring 1991.