As a non-believer I used to really struggle with this question. I'm more mathematical than most people, and the word "eternal" can refer to a mathematically infinite amount of time. (Eventually I'll write a whole web page about the implications of this). The conclusion I came to is that when they wrote the Bible, they didn't know about infinity as understood by modern Western mathematics. They just meant something really, really, really, long.

After many years of reading the Bible, and other texts, and thinking about it, I do believe there really is a hell. Not a literally mathematically eternal one with zero chance of escape — but rather a place that's very very unpleasant (to say the least) and very very difficult to get out of.

The main point being that it's much easier to change your life now, while still alive, than to get stuck in a state of darkness and torment in the afterlife — for who knows how long. Where there will be far, far less opportunity to repent and turn to Christ than there is right now, this very minute, while you're still alive.