This is connected to what “the sign of thy coming” means. It doesn’t mean the second coming of Christ at the very end of the world, it means Christ’s coming to destroy the people who rejected Him as their Messiah! How do I know Jesus returned in 70 AD? Jesus told the high priest Caiaphas He would return!

Matthew 26:63-65 “But Jesus held his peace. And the high priest answered and said unto him, I adjure thee by the living God, that thou tell us whether thou be the Christ, the Son of God. {64} Jesus saith unto him, Thou hast said: nevertheless I say unto you, Hereafter shall ye see the Son of man sitting on the right hand of power, and coming in the clouds of heaven. {65} Then the high priest rent his clothes, saying, He hath spoken blasphemy; what further need have we of witnesses? behold, now ye have heard his blasphemy.”

God has given us an intellect to help us interpret Scripture. Some things are symbolic and others literal. Jesus told the high priest that he would see Him sitting at the right hand of power coming in the clouds of Heaven. It seems logical to me that the high priest would live 40 more years to the destruction of Jerusalem, and literally see Jesus in the clouds, and therefore know that he and the Jews are being judged by God through the Roman army for their rejection of Jesus as their Messiah, their Christ. It therefore can’t possibly mean 2000 years later as some may interpret it.

These scriptures are clearly about the attack by the Roman army by the Jews for their rebellion. The Lord was going to punish the Christ rejecting Jews but did not want His people to be caught up in that judgement. He therefore told His followers to flee Jerusalem and Judea when they saw the Roman army coming. He warned the Christians who were not in Judea not to return to it. He told them to make haste to flee for their lives. He told them to pray it won’t happen in winter when it’s much more difficult to travel, and to pray it won’t be on the Sabbath day when the gates are closed.

Acts 17:10  And the brethren immediately sent away Paul and Silas by night unto Berea: who coming thither went into the synagogue of the Jews.
11  These were more noble than those in Thessalonica, in that they received the word with all readiness of mind, and searched the scriptures daily, whether those things were so.

In the prophetic language, great commotions upon earth are often represented under the notion of commotions and changes in the heavens: –

The fall of Babylon is represented by the stars and constellations of heaven withdrawing their light, and the sun and moon being darkened. See Isaiah 13:9, Isaiah 13:10.

The destruction of Egypt, by the heaven being covered, the sun enveloped with a cloud, and the moon withholding her light. Ezekiel 32:7, Ezekiel 32:8.

The destruction of the Jews by Antiochus Epiphanes is represented by casting down some of the host of heaven, and the stars to the ground. See Daniel 8:10.

And this very destruction of Jerusalem is represented by the Prophet Joel, Joel 2:30, Joel 2:31, by showing wonders in heaven and in earth – darkening the sun, and turning the moon into blood. This general mode of describing these judgments leaves no room to doubt the propriety of its application in the present case.

The falling of stars, i.e. those meteors which are called falling stars by the common people, was deemed an omen of evil times.

Verse 30 Then shall appear the sign of the Son of man – The plain meaning of this is, that the destruction of Jerusalem will be such a remarkable instance of Divine vengeance, such a signal manifestation of Christ’s power and glory, that all the Jewish tribes shall mourn, and many will, in consequence of this manifestation of God, be led to acknowledge Christ and his religion. By της γης, of the land, in the text, is evidently meant here, as in several other places, the land of Judea and its tribes, either its then inhabitants, or the Jewish people wherever found.

Verse 31 He shall send his angels – Τους αγγελους, his messengers, the apostles, and their successors in the Christian ministry.

With a great sound of a trumpet – Or, a loud-sounding trumpet – the earnest affectionate call of the Gospel of peace, life, and salvation.

Shall gather together his elect – The Gentiles, who were now chosen or elected, in place of the rebellious, obstinate Jews, according to Our Lord’s prediction, Matthew 8:11, Matthew 8:12, and Luke 13:28, Luke 13:29. For the children of the kingdom, (the Jews who were born with a legal right to it, but had now finally forfeited that right by their iniquities) should be thrust out. It is worth serious observation, that the Christian religion spread and prevailed mightily after this period: and nothing contributed more to the success of the Gospel than the destruction of Jerusalem happening in the very time and manner, and with the very circumstances, so particularly foretold by our Lord. It was after this period that the kingdom of Christ began, and his reign was established in almost every part of the world.

To St. Matthew’s account, St. Luke adds, Luke 21:24, They shall fall by the edge of the sword, and shalt be led away captive into all nations; and Jerusalem shall be trodden down by the Gentiles, till the times of the Gentiles be fulfilled. The number of those who fell by the sword was very great. Eleven Hundred Thousand perished during the siege. Many were slain at other places, and at other times. By the commandment of Florus, the first author of the war, there were slain at Jerusalem 3,600, Josephus. War, b. ii. c. 14. By the inhabitants of Caesarea, above 20,000. At Scythopolis, above 13,000. At Ascalon, 2,500. At Ptolemais, 2,000. At Alexandria, 50,000. At Joppa, when taken by Cestius Gallus, 8,400. In a mountain called Asamon, near Sepporis, above 2,000. At Damascus, 10,000. In a battle with the Romans at Ascalon, 10,000. In an ambuscade near the same place, 8,000. At Japha, 15,000. Of the Samaritans, on Mount Gerizim, 11,600. At Jotapa, 40,000. At Joppa, when taken by Vespasian, 4,200. At Tarichea, 6,500. And after the city was taken, 1,200. At Gamala, 4,000, besides 5,000 who threw themselves down a precipice. Of those who fled with John, of Gischala, 6,000. Of the Gadarenes, 15,000 slain, besides countless multitudes drowned. In the village of Idumea, above 10,000 slain. At Gerasa, 1,000. At Machaerus, 1,700. In the wood of Jardes, 3,000. In the castle of Masada, 960. In Cyrene, by Catullus the governor, 3,000. Besides these, many of every age, sex, and condition, were slain in the war, who are not reckoned; but, of those who are reckoned, the number amounts to upwards of 1,357,660, which would have appeared incredible, if their own historian had not so particularly enumerated them. See Josephus, War, book ii. c. 18, 20; book iii. c. 2, 7, 8, 9; book iv. c. 1, 2, 7, 8, 9; book vii. c. 6, 9, 11; and Bp. Newton, vol. ii. p. 288-290.

Many also were led away captives into all nations. There were taken at Japha, 2,130. At Jotapa, 1,200. At Tarichea, 6,000 chosen young men, who were sent to Nero; others sold to the number of 30,400, besides those who were given to Agrippa. Of the Gadarenes were taken 2,200. In Idumea above 1,000. Many besides these were taken in Jerusalem; so that, as Josephus says, the number of the captives taken in the whole war amounted to 97,000. Those above seventeen years of age were sent to the works in Egypt; but most were distributed through the Roman provinces, to be destroyed in their theatres by the sword, and by the wild beasts; and those under seventeen years of age were sold for slaves. Eleven thousand in one place perished for want. At Caesarea, Titus, like a thorough-paced infernal savage, murdered 2,500 Jews, in honor of his brother’s birthday; and a greater number at Berytus in honor of his father’s. See Josephus, War, b. vii. c. 3. s. 1. Some he caused to kill each other; some were thrown to the wild beasts; and others burnt alive. And all this was done by a man who was styled, The darling of mankind! Thus were the Jews miserably tormented, and distributed over the Roman provinces; and continue to be distressed and dispersed over all the nations of the world to the present day. Jerusalem also was, according to the prediction of our Lord, to be trodden down by the Gentiles. Accordingly it has never since been in the possession of the Jews. It was first in subjection to the Romans, afterwards to the Saracens, then to the Franks, after to the Mamalukes, and now to the Turks. Thus has the prophecy of Christ been most literally and terribly fulfilled, on a people who are still preserved as continued monuments of the truth of our Lord’s prediction, and of the truth of the Christian religion. See more in Bp. Newton’s Dissert. vol. ii. p. 291, etc.

My commentary of This generation shall not pass away: Jesus was talking to his disciples who were mostly young. They would live 40 more years to see all these things, namely the destruction of the Temple and Jerusalem by the Roman army.

I hope you see these verses are not talking about the Rapture as the popular “Left Behind” series by Tim LaHaye indicates. It’s talking about the killing of Jews by the Romans.