I'll certainly unpack this for you. Although I would not trust that internet reading alone would give anybody a comprehensive understanding of the Jewish religion (or any other religion, for that matter), you can read about Jewish conceptions of the afterlife here, for example: http://www.jewfaq.org/olamhaba.htm. There is no single doctrine to which Jews adhere regarding the afterlife (in fact, many Jews do not have any defined concept of an afterlife at all). What I would like to propose is that Jesus' words that do seem to reference a state of exile or suffering may not be doctrinal, but rather metaphorical in nature. Whether this state is an external, otherworldly one or an internal state of mind / soul in this life is a matter of debate. I should mention here that the "traditional" Christian doctrine of an otherworldly heaven and hell has been linked by scholars to Zoroastrianism--a religion which strongly influenced some types of early Christianity as well as Judaism. Therefore, a question: how do we know that orthodox Christian doctrines are not the outgrowth of historical misunderstandings / misinterpretations of Jesus' words? To further complicate things, you have the entirety of the Gnostic scriptures to deal with (which include some of the earliest known Christian writings), and if, after reading them, you do not find your traditional understanding of Christian doctrines disturbed, you probably aren't reading closely enough.
You can read about Gehenna, Zoroastrianism, and Gnosticism on Wikipedia or do some scholarly / historical research on your own, and thus come to your own conclusions. There is a gigantic body of research already done regarding these matters, and I would not consider any list of books that I could provide comprehensive enough to do this body of work justice. In addition, please do not simply accept my, or anybody's, answers to these questions without thinking for yourself and doing some investigation into them on your own.
I'm glad that Matthew 7: 17-23 was brought up. Here are the verses:
"Not everyone who says to me, 'Lord, Lord,' will enter the kingdom of heaven, but only the one who does the will of my Father in heaven. Many will say to me on that day, 'Lord, Lord, did we not prophesy in your name? Did we not drive out demons in your name? Did we not do mighty deeds in your name?' Then I will declare to them solemnly, 'I never knew you. Depart from me, you evildoers.'"
So, AffableGeek, is it possible to say that, for those Jesus is speaking about here, simple belief in Jesus/God does not suffice to allow entrance into the "kingdom of heaven"? They did all kinds of miracles in Jesus' name and called upon him as their "Lord," but still, they did not actively "do the will of [Jesus'] Father in heaven" and so they do not enter into the kingdom of heaven.
What might it mean to enter into the kingdom of heaven? Let's look at another passage. Here is Luke 17:20:
"Asked by the Pharisees when the kingdom of God would come, he [Jesus] said in reply, 'The coming of the kingdom of God cannot be observed, and no one will announce, 'Look, here it is,' or 'There it is.’ For behold, the kingdom of God is within you."
Jesus does not say that the kingdom of God is external to you or that it is a destination that one reaches only after physical death, but that it is within you--present in this very life at this very moment. Perhaps, then, it would be wise to consider that both "heaven" and "hell" as they are traditionally conceived (as eternal, external states of being) are, in actuality, inner states of mind or soul that are generated by actions in this life. In short, perhaps these notions are only metaphors, as I stated above.
In this light, let’s consider the beginning of the Gospel of Thomas:
“Jesus said, If your leaders say to you, ‘Behold, the kingdom is in the sky,’ then the birds in the sky will get there before you. If they say to you, ‘It is in the sea,’ then the fish will get there before you. Rather, the kingdom is inside you and outside you. When you know yourselves, then you will be known, and will understand that you are children of the living Father. But if you do not know yourselves, then you live in poverty, and embody poverty” (Saying 3).
Jesus is never clear about the existential ontology of a physical “heaven” or “hell,” but he is clear about other more important issues. He is clear about the fact that love is that which is most central to his message, not judgment. In answer to a scholar of the law who asks Jesus, “Teacher, what must I do to inherit eternal life?” Jesus asks him, “What is written in the law? How do you read it?” The scholar (not Jesus himself) answers,
“You shall love the Lord, your God, with all your heart, with all your being, with all your strength, and with all your mind, and your neighbor as yourself.” Jesus replies to him, “You have answered correctly; do this and you will live” (Luke 10: 25-28).
To answer to AffableGeek's question regarding John 3:18--please note that these are not Jesus' words, but the words of whomever it was who wrote the Gospel of John (and whoever that person was, it was certainly not the apostle John himself. Contemporary scholars know that none of the four traditional Gospels were written by those whose name they bear.)
The initial question was whether or not Jesus actually ever said "If you don't believe in me you go to hell." Did he say these words exactly, ever? If I am missing something, please let me know. Yes, the author of the Gospel of John says something approximate to this (depending on how you interpret his words), and yet Jesus never did himself. And yes, Jesus is said to have spoken the following words after the resurrection, according to Mark 16:16:
“Whoever believes and is baptized will be saved; whoever does not believe will be condemned.”
However, as I have already shown above, Matthew 7:17-21 refutes the notion that mere faith suffices for entrance into the kingdom of heaven. Another question: how do we know that the passage from Mark’s Gospel (or any other passages in the bible, Gnostic scriptures, etc. for that matter) quotes Jesus directly? Like most other accounts of episodes in Jesus’ life, resurrection accounts greatly contradict one another.
If the Official Doctrine of Hell was so important to Jesus himself (like it is to so many fundamentalists), why didn't Jesus explicitly say that one must believe in him and accept him as one’s “personal savior” to get to “heaven” while he was alive over and over again? Also, if we are to take everything written in the bible as absolute truth, we should also, for example, wholeheartedly support slavery, since Paul tells us that slavery is a natural state of being (Ephesians 6:5). There are a great number of other horrible, violent, and/or nonsensical “dogmas” that the writers of the books of the bible advance as well. Must we accept all of them unquestioningly, and return to a medieval state of existence in which—for example—our “women” are considered untouchable during their menstrual cycles, as Leviticus 15: 19-30 dictates?
There is much more to say about all these matters, and much more evidence to be considered that I cannot speak of here due to space and time issues. Please do reading on your own, and don't expect the internet or other people to answer such important questions. If you're serious about an intellectual foray into questions of this nature, sign up for some university courses about religion or do reading on your own, as I said. It may be that you'll be surprised at what you can discover.
Peace to all of you.