Great Tribulation-
This was the occasion of the first persecution; and the barbarities exercised on the Christians were such as even excited the commiseration of the Romans themselves. Nero even refined upon cruelty, and contrived all manner of punishments for the Christians that the most infernal imagination could design. In particular, he had some sewed up in skins of wild beasts, and then worried by dogs until they expired; and others dressed in shirts made stiff with wax, fixed to axletrees, and set on fire in his gardens, in order to illuminate them. This persecution was general throughout the whole Roman Empire; but it rather increased than diminished the spirit of Christianity. In the course of it, St. Paul and St. Peter were martyred.
The First Persecution, Under Nero, A.D. 67 - Fox's Book of Martyrs
Among the numerous martyrs that suffered during this persecution was Simeon, bishop of Jerusalem, who was crucified; and St. John, who was boiled in oil, and afterward banished to Patmos. Flavia, the daughter of a Roman senator, was likewise banished to Pontus; and a law was made, "That no Christian, once brought before the tribunal, should be exempted from punishment without renouncing his religion." A variety of fabricated tales were, during this reign, composed in order to injure the Christians. Such was the infatuation of the pagans, that, if famine, pestilence, or earthquakes afflicted any of the Roman provinces, it was laid upon the Christians.
The Second Persecution, Under Domitian, A.D. 81 - Fox's Book of Martyrs
In the third persecution Pliny the Second, a man learned and famous, seeing the lamentable slaughter of Christians, and moved therewith to pity, wrote to Trajan, certifying him that there were many thousands of them daily put to death, of which none did any thing contrary to the Roman laws worthy of persecution.
The Third Persecution, Under Trajan, A.D. 108 - Fox's Book of Martyrs
The Fourth Persecution, Under Marcus Aurelius Antoninus, A.D. 162
Marcus Aurelius, followed about the year of our Lord 161, a man of nature more stern and severe; and, although in study of philosophy and in civil government no less commendable, yet, toward the Christians sharp and fierce; by whom was moved the fourth persecution.
The cruelties used in this persecution were such that many of the spectators shuddered with horror at the sight, and were astonished at the intrepidity of the sufferers. Some of the martyrs were obliged to pass, with their already wounded feet, over thorns, nails, sharp shells, etc. upon their points, others were scourged until their sinews and veins lay bare, and after suffering the most excruciating tortures that could be devised, they were destroyed by the most terrible deaths.
The Fourth Persecution, Under Marcus Aurelius Antoninus, A.D. 162 - Fox's Book of Martyrs
The Fifth Persecution, Commencing with Severus, A.D. 192 - Fox's Book of Martyrs
https://www.biblestudytools.com/hist...s-a-d-235.html
https://www.biblestudytools.com/hist...s-a-d-249.html
https://www.biblestudytools.com/hist...n-a-d-257.html
https://www.biblestudytools.com/hist...n-a-d-274.html
https://www.biblestudytools.com/hist...n-a-d-303.html
And there is so much more historical data for anyone interested in watching it all unfold.
https://www.biblestudytools.com/hist...n-a-d-303.html
These are the things Jesus was warning them about.