This generation shall not pass away

 

At least on the face of it, Jesus promised that he would return during the lifetime of his apostles. That is, at some point during the first century. Paul clearly had this belief. And on the authority of 2 Peter, we know that many other Christians did, too. 

Two attempts to cope with this problem are known as Partial Preterism and Full Preterism (or Hyper-Preterism). The Preterists believe that Jesus was speaking of an invisible "second advent" that took place in AD 70, when the Romans destroyed Jerusalem and the Second Temple - a cataclysmic historical event indeed mentioned in the Gospels (by way of supposed prophecy). Full Preterism doesn´t believe in a future physical second (third?) advent at all. 

The case for Preterism is stronger than I expected, but I don´t think it explains everything, thus still leaving Christians vulnerable to the criticism that Jesus failed to appear on schedule. The "Futurists" of course exegete the contentious verses in their own ways, but how convincing is that? My guess is that earliest Christianity really had a very strong apocalyptic streak, and like in all similar cases, the apocalypse had to be postponed (Luke, Acts, 2 Peter), played down (the Gospel of John) or allegorized (as in modern hyper-preterism) when it didn´t arrive on schedule. Which would make Jesus just another failed prophet (from an atheist perspective - since prophecy is impossible anyway) or a false one (from a Jewish perspective). 

The links above goes to one Futurist presentation, featuring William Lane Craig, one defense of Preterism, and one "neutral" video which summarizes the Preterist position well enough. 

There are all kinds of problems with Christianity, but it would be *somewhat* more convincing if it hadn´t been for the failed prophecies. And even if we assume that the prophecies were supposed to mean something else, why couldn´t the Holy Ghost explain the subtleties of scriptural exegesis to the early Christians? They supposedly got revelations through the Spirit all the time, funny they somehow missed that the apocalypse was still in the distant future...