“Say what you like,” we shall be told, “the apocalyptic beliefs of the first Christians have been proved to be false. It is clear from the New Testament that they all expected the Second Coming in their own lifetime. And worse still, they had a reason, and one which you will find very embarrassing. Their Master had told them so. He shared, and indeed created, their delusion. He said in so many words, ‘this generation shall not pass till all these things be done.’ And he was wrong. He clearly knew no more about the end of the world than anyone else.”
It is certainly the most embarrassing verse in the Bible. Yet how teasing, also, that within fourteen words of it should come the statement “But of that day and that hour knoweth no man, no, not the angels which are in heaven, neither the Son, but the Father.” The one exhibition of error and the one confession of ignorance grow side by side. That they stood thus in the mouth of Jesus himself, and were not merely placed thus by the reporter, we surely need not doubt. Unless the reporter were perfectly honest he would never have recorded the confession of ignorance at all; he could have had no motive for doing so except a desire to tell the whole truth. And unless later copyists were equally honest they would never have preserved the (apparently) mistaken prediction about “this generation” after the passage of time had shown the (apparent) mistake. This passage (Mark 13:30—32) and the cry “Why hast thou forsaken me?” (Mark 15:34) together make up the strongest proof that the New Testament is historically reliable. The evangelists have the first great characteristic of honest witnesses: they mention facts which are, at first sight, damaging to their main contention.
C.S. Lewis, "The World's Last Night" (1960)