Luke omits Jesus' prediction at his trial that his Jewish judges would see the Son of Man coming in the clouds. Luke also omits that they would see him seated at the right hand.
These predictions are made at Jesus' trial as found in Mark and in Matthew but not in Luke:
And Jesus said, I am: and ye shall see the Son of man sitting on the right hand of power, and coming in the clouds of heaven.
-- Mark 14:62
Jesus said to him, "You have said it yourself. But I tell you, from now on [ἀπ᾽ ἄρτι] you will see the Son of Man sitting at the right hand of the Power and coming on the clouds of heaven." -- Matthew 26:64
But from now on [ἀπὸ τοῦ νῦν] the Son of man shall be seated at the right hand of the power of God.
-- Luke 22:69
Of course, some commentators get around the omissions by positing that Luke simply used a different, independent source from Mark and Matthew at this point, but that simply leaves us with two competing versions of what Jesus said.
Luke, however, is not unaware of the main idea and has Jesus say it elsewhere, and therefore it is not necessary to posit a different source but that he has simply made a different editorial decision about where and when to put it. To Luke it doesn't belong at the trial.
Like Mark 13:26 and Matthew 24:30, who thus have the conception uttered twice by Jesus, Luke reserves it to his version of the Little Apocalypse about the end of the world, where "they" refers to humanity in general:
And then shall they see the Son of man coming in a cloud with power and great glory.
-- Luke 21:27
This makes more sense to Luke, and removes what looks like a difficulty for him if Matthew and Mark are insisting what they appear to be insisting.
For Luke the kingdom is already here because Jesus is present and working (Luke 17:20f.), but it will never really be "at hand" as it is in Matthew (3:2; 4:17; 10:7) and Mark (1:15) until a little later, when the trees shoot forth in the summer (Luke 21:30f.). For Luke's apocalyptic Jesus, the appearance of such leaves is analogous to the emergence of the signs of the end of the world in sun, moon, and stars: chaos on land and sea and the powers of heaven rocked (Luke 21:25f.).
In Luke's hands Jesus now states perfunctorily at his trial that the Son of Man will sit at God's right hand, dropping the coming on the clouds and the prediction that his Jewish judges will see that or the enthronement. For good reason. Presumably he knows that Annas and Caiaphas died in the 40s and lived to see nothing, and Luke as he is writing has not witnessed the fulfillment of such predictions either.
It is little appreciated how Luke's editorial activity in the trial scene is connected to his larger theological project.
It is designed to agree with
Luke's understanding of Jesus exalted at God's right hand in Acts, continuing his presence on earth by directing the missionary activities of the church through the Spirit, especially those of Paul among the Gentiles.
Jesus' Jewish judges are now completely beside the point. God has bypassed them, just has Paul and Barnabas shook off the dust from their own feet against the Jews at Pisidian Antioch and turned to the Gentiles instead (Acts 13).
For Luke, the judgment of the Jews is postponed temporarily until the still imminent but delayed end of the world, when Jesus will then bring vengeance upon Judea (Luke 21:22, 31).
God's focus is turning elsewhere in the meantime. Jesus' objective is no longer his immediate return for the judgment of Israel, but rather a near-term future of reigning at the right hand of power in order that the whole world might repent and be saved (Acts 2:39; John 3:17; Romans 4:16; 16:26; I Corinthians 9:22; I Timothy 2:4; Titus 2:11; II Peter 3:9).
Luke clearly thinks Mark and Matthew have the trial details wrong, just as they have wrong the reason for Jesus' trial (Jesus' call to discipleship required radical poverty, a direct threat to the revenue of the Jewish temple, and so to the Roman treasury). Jesus is no longer returning immediately to turn the tables on his Jewish judges, to become the judge instead of the judged. He is remaining at God's right hand to do something else: extend God's offer of mercy to all of mankind.
Consequently the coming of the Son of Man on the clouds of heaven for Luke is now a matter of a future second coming, conforming to a more or less structured apocalyptic narrative, unfolding at an undetermined but still imminent point in the near future, in agreement with the apocalyptic parallel narratives of both Mark and Matthew.
And then (καὶ τότε) shall they see the Son of man coming in a cloud with power and great glory.
-- Luke 21:27
And then shall they see the Son of man coming in the clouds with great power and glory.
-- Mark 13:26
And then shall appear the sign of the Son of man in heaven: and then
shall all the tribes of the earth mourn, and they shall see the Son of
man coming in the clouds of heaven with power and great glory.
-- Matthew 24:30
The imminently coming eschatological Son of Man without signs still front and center in Jesus' mind at his trial according to Mark and Matthew has been relegated to a future second coming narrative of his followers creation.
It is easier to explain the development of the Little Apocalypses of the gospels as derivative from an original, simple, and straightforward eschatological belief than it is the other way around. The former was developed in an elaborate manner to explain the failure of the latter.
Those narratives notably all have Jesus condescend to address an apocalyptic timetable which was anathema to the original eschatological message, supplying a second coming replete with signs in the heavens above and the earth below which indicate that the ensuing end of the world can indeed be said to be observable to a certain extent, despite the fact that Jesus had in no uncertain terms eschewed any such observable signs, most notably in Mark 8:12:
There shall no sign be given unto this generation.
Luke is not unaware of this tradition, either:
And when he was demanded of the Pharisees, when the kingdom of God
should come, he answered them and said, The kingdom of God cometh not
with observation: Neither shall they say, Lo here! or, lo there! for, behold, the kingdom of God is within you [plural Pharisees].
-- Luke 17:20f.
The kingdom was already there among them, in the person of Jesus, and they had already missed it. It did not need Jesus to die and rise to be present. There would be no apocalyptic signs. It had already come as a surprise without them. Repent and follow him or perish!
But as both Luke and Matthew hedge Mark on Jesus' trial statements (Matthew followed by Luke already extenuate by adding "from now on", see above), they both hedge Mark about the signs as well, supplementing Mark 8:12 in their parallels with "no sign but the sign of the prophet Jonah" who was three days and three nights in the belly of the fish, about whom Mark knows . . . nothing (Matthew 12:39; 16:4; Luke 11:29f.).
It is clear what is going on here.
Matthew and Luke reinterpret what is ostensibly the earliest tradition from the point of view of the resurrection wherever they can, freely tampering, dare we say it, with the word of God (II Corinthians 4:2) just as much as Mark had done (for example, by making Jesus' predict his rising on the third day in Mark 8:31; 9:31; 10:34; 14:28). They are, all of them, to one degree or another, with one degree of success or another, the new scribes of the kingdom of heaven (conveniently provided for by the kingdom-as-net story in Matthew 13:52 to justify their activity) who bring out of their treasure things new and old, discarding the bad and keeping the good.
The death of Jesus required as much. This bad thing that happened to Jesus had to be explained. They thought he would bring the kingdom and he did not.
In the case of the NT apocalyptic narratives, which portray Jesus willingly and volubly engaging in talk of signs of the end of the world with the disciples, Jesus' future return as the Son of Man is now predicated on the gospel first being published among all the nations (Mark 13:10), until the times of the Gentiles are fulfilled (Luke 21:24), so that all nations hear and come to hate the elect, original disciples (Matthew 24:9, 14). At which point all the tribes of the earth shall mourn when they see the Son of Man return in the clouds of heaven because judgment is finally nigh. Go ye therefore and make disciples of all nations while there is still time (Matthew 28:19f.).
In this the gospels overwhelmingly evidence the new point of view of the church, especially championed by Luke in Acts, which ends with Paul's arrival in Rome, the center of the world (The epistles still teem with apocalyptic expectation because with that achievement, it's mission accomplished).
Gone is the high dudgeon of the Jesus who said only an "evil generation" seeks after a sign (Matthew 12:39; Matthew 16:4; Luke 11:29).
All of it flies in the face of Jesus' command to go not into the way of the Gentiles (Matthew 10:5f.), and of a host of other awkward eruptions of the original, simple eschatology in the halfway houses of the evangelists:
that he was sent only to the lost sheep of the House of Israel (Matthew 15:24),
that his followers would judge the twelve tribes of Israel, not Gentiles (Matthew 19:28; Luke 22:30),
that those followers will not have gone over the cities of Israel till the Son of man be come (Matthew 10:23),
that the kingdom is at hand (Matthew 3:2; Matthew 4:17; Matthew 10:7; Matthew 26:18, 45; Mark 1:15; Luke 10:9, 11),
that the kingdom is already present in exorcisms (Matthew 12:28; Luke 11:20),
that the Son of Man would come in his kingdom before the deaths of some of the disciples (Matthew 16:28; Mark 9:1; Luke 9:27; John 21:23),
that the kingdom is already in their midst but is unobserved (Luke 17:20-21),
and that there was a general buzz of expectation around Jesus that the kingdom was coming immediately in Jerusalem for some reason (Luke 19:11), an expectation most especially embraced by Jesus' own disciples even until the very last when Jesus ascended into heaven (Acts 1:6).
But the resurrection? They were supposedly blind to the very idea of it to the end and beyond. The resurrection "they yet believed not . . ." (Luke 24:41)! But a kingdom restored to Israel, that they most certainly did believe to the end and beyond, but wrongly!
Where oh where did they get that idea, if not from Jesus? The historical Jesus preached the imminently coming kingdom, an idea they did have, not the resurrection, an idea they did not.
The apocalyptic narratives are a mixture of the complicated, rationalized new and the simple, enthusiastic old. They contain at the same time 1) a thought out timetable with signs for the end of the world which was anathema to Jesus and 2) a memory of the unpredictable in-breaking of the kingdom which has no timetable, the message he actually preached.
It was the latter which otherwise and everywhere occasioned all this urgency and expectation swirling about Jesus in the first place.
His simple conception of the unpredictable end of the world, without apocalyptic adornment, is best remembered only by Matthew:
Then Jesus sent the multitude away, and went into the house: and his
disciples came unto him, saying, Declare unto us the parable of the
tares of the field. He answered and said unto them, He that soweth the good seed is the Son of man;
The field is the world; the good seed are the children of the kingdom; but the tares are the children of the wicked one;
The enemy that sowed them is the devil; the harvest is the end of the world; and the reapers are the angels.
As therefore the tares are gathered and burned in the fire; so shall it be in the end of this world.
The Son of man shall send forth his angels, and they shall gather out of
his kingdom all things that offend, and them which do iniquity;
And shall cast them into a furnace of fire: there shall be wailing and gnashing of teeth.
Then shall the righteous shine forth as the sun in the kingdom of their Father. Who hath ears to hear, let him hear.
-- Matthew 13:36ff.
Mark and Matthew tell us that Jesus believed this even to his fateful end:
"Ye shall see the Son of Man coming in the clouds of heaven".