Albert Schweitzer, answered from the texts and cited to the page.
The delay of the Parousia is simply this: Jesus expected the end, and it did not come. When he sent the Twelve out on their mission, he was not dispatching them on an ordinary preaching tour. The whole discourse at the sending forth, taken in its plain sense, is a prediction of the events of the time of the end — the sufferings, the outpouring of the Spirit, the appearance of the Son of Man — all of it understood as immediately at hand.1
He told them plainly that they would not have gone through the cities of Israel before the Son of Man came.2 He did not expect to see them again on this side of the Kingdom. They returned. The sufferings had not come, the Spirit had not been poured out, the Son of Man had not appeared.3 What came back to Jesus was not the end of the world but a group of disciples, somewhat proud of their power over demons, otherwise intact.
That return was, for Jesus, the first collision between the dogmatic history he had tried to impose on reality and actual history — and actual history won.4 The consequences of that collision run through everything that follows. The problem of the delay did not end with Jesus; it became the central problem of primitive Christianity and then of the whole subsequent development of Christian belief.
Already in Paul's time it was pressing enough that he had to manufacture reasons in 2 Thessalonians for why the Second Coming had not yet arrived.5 The author of 2 Peter resorted to the sophism that a thousand years are in God's sight as a single day — a device that ignores the fact that the original promise was reckoned in human years, not divine ones.6
Montanism, a century later, was essentially a revolt against the progressive domestication of the hope, an insistence that the nearness of the Kingdom be taken with the same seriousness it had in primitive Christianity.7 The whole inner history of Christianity, from the first generation onward, is the history of what happens when communities built on an imminent expectation are forced to go on living after the expectation is not met.8
De-eschatologizing — the progressive stripping away of the apocalyptic framework — is not a later theological refinement; it is the institutional response to a specific, datable disappointment. The non-fulfillment of the promise in Matthew 10:23 is, in that sense, the first significant date in the history of Christianity.9
The whole of the discourse at the sending forth of the Twelve, taken in the clear sense of the words, is a prediction of the events of the 'time of the end,' events which are immediately at hand, in which the supernatural eschatological course of history will break through into the natural course.The Quest of the Historical Jesus
Schweitzer was convinced that 'at the time of their mission,' Jesus 'did not expect them to return before the Parousia' (386).Reverence for Life, The Ethics of Albert Schweitzer, pp. 269–270
There followed neither the sufferings, nor the outpouring of the Spirit, nor the Parousia of the Son of Man. The disciples returned safe and sound and full of a proud satisfaction; for one promise had been realized—the power which had been given them over the demons.Reverence for Life, The Ethics of Albert Schweitzer, pp. 269–270
It is equally clear, and here the dogmatic considerations which guided the resolutions of Jesus become still more prominent, that this prediction was not fulfilled. The disciples returned to him; and the appearing of the Son of Man had not taken place. The actual history disavowed the dogmatic history on which the action of Jesus had been based.Reverence for Life, The Ethics of Albert Schweitzer, pp. 269–270
Already in Paul's time the problem was pressing, and he had to set to work in 2 Thessalonians to discover all possible and impossible reasons why the Second Coming should be delayed.The Quest of the Historical Jesus
The author of 2 Peter has a much clearer notion of what he would be at, and undertakes to restore the confidence of Christendom once for all with the sophism of the thousand years which are in the sight of God as one day, ignoring the fact that in the promise the reckoning was by man's years, not by God's.The Quest of the Historical Jesus
This enthusiastic movement, which began in Mysia, was determined to give to the belief in the immediate nearness of the Kingdom the same importance as it had had in Primitive Christianity.The Mysticism of Paul the Apostle, pp. 357–358
The whole history of 'Christianity' down to the present day, that is to say, the real inner history of it, is based on the delay of the Parousia, the non-occurrence of the Parousia, the abandonment of eschatology, the progress and completion of the 'de-eschatologizing' of religion which has been connected therewith.Reverence for Life, The Ethics of Albert Schweitzer, pp. 269–270
The non-fulfillment of Matt. x. 23 is the first postponement of the Parousia. We have therefore here the first significant date in the 'history of Christianity'; it gives to the work of Jesus a new direction, otherwise inexplicable.Reverence for Life, The Ethics of Albert Schweitzer, pp. 269–270