In order to understand the importance of the Resurrection, we first have to understand what the first-century views of resurrection were. What were the prior expectations of the people who first heard the story of Jesus' resurrection?
In the non-Jewish Greco-Roman world, there were two predominant views of the afterlife. The first was the basic materialist stance that there is no afterlife. The Greek Epicurean philosophical school would be an example of this stance. Although construed somewhat differently than contemporary philosophers and scientists would use the terms, this point of view would hold that the material universe is all there is. There is no "spirit" apart from the body in which there is any consciousness, hence there is no continuing existence once the physical body ceases to function. Dead is dead.
The other major strand of Greco-Roman thought is well represented by Plato. There is a continued existence after death; in fact, this is what earthly existence longs for--a release from bondage to the body and the corrupted physical realm. We are essentially spirits trapped in bodies, longing for release. You may recognize this point of view--many Christians' view of "heaven" owes more to Plato than to the Bible. This earthly life is something merely to be endured until we escape it to live in heaven forevermore with God. More on this in another post.
For now, the relevant point is that no one in the Greco-Roman world was expecting anything like physical resurrection. It would either have been considered impossible or pointless. The spirit either didn't exist apart from the physical body, or if it did, the last thing it wanted was to be re-embodied. (The concept of reincarnation did exist, but this is different from resurrection--it is embodiment in a different body, not the same one, and was not considered the goal of existence, but rather a punishment or a continued stage on the way to fully-realized--that is, disembodied--spirituality.)
But that's the non-Jewish world. Christianity arose in a Jewish cultural context. What did the Jews believe regarding resurrection?
Once again, there were two predominant views. And once again, one of them precluded resurrection. The Sadducean group, which dominated the Temple priesthood, rejected resurrection (as well as angels and providence). While present-day Christians tend to scoff at this point of view, it actually accords with the Sadducees' generally more strict reading of the Hebrew scriptures, which must be admitted to have little to say on the afterlife in general and resurrection specifically.
On the other hand, the Pharisaical group which dominated the rabbis in the synagogues did believe in resurrection. However, this resurrection was not expected to occur until the eschaton--the final culmination of history. After the death of Lazarus in John 11, his sister Martha expresses this point of view in her dialogue with Jesus: "Martha answered, 'I know he will rise again in the resurrection at the last day'." So yes, there were Jews who believed in resurrection, but not in present-day reality--only at the end of history. It's also worth noting that resurrection played no part in Jewish messianic expectations. Messiah was to bring about the liberation of Israel as a nation and reestablish the throne of David; neither the death of the messiah nor a resurrection was envisioned.
What does this all mean with regard to Jesus' resurrection? Quite simply, it means that the usual explanations for why Jesus' resurrection is important are wrong, or at least beside the main point.
Christians generally view Jesus' resurrection in terms of God vindicating Jesus, demonstrating that he was the Messiah, God in flesh, and innocent of any crime or sin for which he should die. But while this all is true, it reflects backward reasoning: if we come to trust in Jesus and believe that he was God in human flesh, then his resurrection takes on all these meanings. But resurrection itself would not have demonstrated any of these things to anyone in the first-century world. Remember, nobody was expecting anyone to be resurrected--not in the Gentile world at all, and not in the Jewish world in the here-and-now.
But for those who were hoping for resurrection "at the last day," as Martha was, Jesus' resurrection would have meant something mind-blowing and worldview-changing: that something that had been hoped for at the end of time was a present reality, right now. It was a glimpse into a deeply longed-for future, breaking into the present. Evidently, when Jesus (and before him, John the Baptist) were proclaiming that "the kingdom of heaven is at hand" (Matthew 3:2; 4:17), it really was.
Everything they had ever hoped for was beginning to come to pass. Right now.
That's what Jesus' resurrection meant. What it implies, for those who believe, is where we're headed next.
[HT: N.T. Wright, Surprised by Hope.]