One of the main areas of scripture to which Schweitzer pointed was Matthew 24; but if we look closely at that chapter, we see that after Christ listed many things that would happen at the end of the age, he said:
“Now learn this lesson from the fig tree: As soon as its twigs get tender and its leaves come out, you know that summer is near. Even so, when you see all these things, you know that it is near, right at the door. Truly I tell you, this generation will certainly not pass away until all these things have happened. Heaven and earth will pass away, but my words will never pass away. But about that day or hour no one knows, not even the angels in heaven, nor the Son, but only the Father” (Matthew 24:32-36).
First, we should realize that Jesus’ words regarding “this generation” may simply mean that the generation Christ was speaking of – the generation that would witness the signs he said would occur – would not pass away till the signs were all fulfilled and the end occurred. The Greek pronoun translated "this" can often be translated "the same" and we should keep this meaning in mind.
It is clear that some of the sayings of Jesus recorded in Matthew 24 found fulfillment in AD 70 with the Roman destruction of the Temple in Jerusalem (Matthew 24:1-2), but other statements contained in his discourse on the Mount of Olives in that same chapter have a clear setting in a distant future, as we see, for example, in the prophecies: “And this gospel of the kingdom will be preached in the whole world as a testimony to all nations, and then the end will come” (Matthew 24:14), and “For then there will be great distress, unequaled from the beginning of the world until now—and never to be equaled again” (Matthew 24:21) – neither of which were fulfilled in AD 70.
Schweitzer also appealed to Matthew 23 as a chapter he thought showed Jesus taught the end would occur at that time, but there, in his criticism of the teachers of the law and Pharisees, Jesus simply said:
“You snakes! You brood of vipers! How will you escape being condemned … And so upon you will come all the righteous blood that has been shed on earth, from the blood of righteous Abel to the blood of Zechariah son of Berekiah, whom you murdered between the temple and the altar. Truly I tell you, all this will come on this generation” (Matthew 23:33-36).
In this instance, Jesus stated that the guilt of the religious leaders who had persecuted and killed many of God’s servants would not escape them, and that punishment would come on that generation of leaders who were no different from their ancestors. Because the Romans destroyed the Temple and killed many of the religious leaders in AD 70, that prediction effectively came to pass.
Other key verses that Schweitzer believed showed Jesus promised his return in his own day are: “There be some standing here which shall not taste of death till they see the Son of man coming in his kingdom” (Matthew 16:28); “... until they see that the kingdom of God has come with power” (Mark 9:1); and “... till they see the kingdom of God” (Luke 9:27). But these parallel accounts all record Jesus’ words being spoken immediately before the Gospels record the Transfiguration – in which some (as Christ said) of the disciples saw Jesus in a vision of divine splendor which was a "preview" of Christ in the Kingdom of God.
Although it is clear historically that many early Christians believed that Jesus would return in their lifetimes, the words of Christ and the apostles always focused not on the timing of the end, but on its imminence. We must all live in readiness for the Kingdom of God - not only because we do not know the time it will be fully instituted at the return of Christ, but also because we do not know when our own lives will end.