Opponents of universalism often cite specific proof-texts, for the New Testament is clear that there are people who will go to hell. Jesus says in Matthew 25:46, "And these will go away into eternal punishment, but the righteous into eternal life" (NRSV). In Mark 9:43-46, Jesus repeatedly calls hell a place where the fire shall never be quenched. And Revelation 20:10 implies that hell is a place of everlasting torment, for it states, "And the devil who had deceived them was thrown into the lake of fire and sulfur, where the beast and the false prophet were, and they will be tormented day and night forever and ever."
The Greek word translated "eternal" and "forever" is from aion, and it indeed can mean eternity. Its Hebrew equivalent is olam, which also can carry the connotation of "forever." Olam is used when the Bible says that God's mercy endures forever, or that God's anger will not last forever, or that Israel will dwell in her land forever (and the Septuagint translation of such passages uses some form of aion). These passages are so assuring because they carry a notion of perpetuity.
But aion and olam do not imply eternity in every case.
For example, Jeremiah 18:15-16 states: "But my people have forgotten me, they burn offerings to a delusion; they have stumbled in their ways, in the ancient roads, and have gone into bypaths, not the highway, making their land a horror, a thing to be hissed at forever. All who pass by it are horrified and shake their heads."
Jeremiah 23:39-40 has: "I will surely lift you up and cast you away from my presence, you and the city that I gave to you and your ancestors. And I will bring upon you everlasting disgrace and perpetual shame, which shall not be forgotten."
Describing God's wrath upon Zion, Isaiah 33:14 states: "The sinners in Zion are afraid; trembling has seized the godless: 'Who among us can live with the devouring fire? Who among us can live with everlasting flames?'"
Or take the statement that the fires of hell will not be quenched. Jeremiah 7:20 says about Jerusalem: "Therefore thus saith the Lord; Behold, my anger and wrath shall be poured out upon this place, and upon the men, and upon the cattle, and upon every tree of their field, and upon the fruits of the land; and it shall burn, and not be quenched."
In these passages, we see the theme of eternal punishment, or of an everlasting fire, or of a fire that cannot be quenched. And these are all referring to God's punishment of Israel and/or Jerusalem. Yet, we know that the same prophets also describe God's ultimate restoration of his nation and his city. God says that they'll be destroyed forever, but that's not exactly what happens. So I guess that his eternal punishment was not eternal after all, at least not in that case.
Or take Jonah 2:6: "I went down to the land whose bars closed upon me forever; yet you brought up my life from the Pit, O LORD my God." Jonah says that the bars were around him forever (olam), yet God delivered him from them.
So what do I do with all this? The Bible says that God will punish people eternally. But what if "eternal" or "everlasting" or "forever" in such cases simply means "for a very long time"? Do those passages absolutely have to deprive us of all hope--for our unsaved loved ones who died before saying the sinner's prayer, or for those who never heard the Gospel, or for the people who heard it yet did not fully receive it (for whatever reason)?
A lot of hard-core fundamentalists and evangelicals will answer, "But the Bible says they'll burn forever and ever! It offers no hope for them after they die!" Well, God told the Israelites in Judges 10:13-16 that he would deliver the Israelites no more, but he changed his mind after they repented (or at least appeared to do so). Why should we assume that God's absolutely inflexible about his words?
At the same time, it's important to understand that this is all speculation. Maybe "eternal punishment" literally does mean "eternal punishment." One thing that gets on my nerves about universalists is that they are SO dogmatic. In many cases, universalism practically is their religion. One of my relatives bases many of his beliefs on the idea that "God will convert everybody anyhow."
That's a lot of weight to place on a doctrine that's not explicitly in the Bible! The Bible doesn't put near the amount of emphasis on post-mortem salvation that its adherents do (actually, the Bible doesn't even seem to mention it). All I'm saying is that there might be some window of hope in the Scriptures for those who die in an unsaved state, so we shouldn't completely despair if that happens to our friends or loved ones.
But we shouldn't play with God either, for the Bible talks a lot about punishment. And a huge part of our witness should be warning people to flee from the wrath to come.