Concise Bible Dictionary:

Besides the application of this term to any time of distress, and its special reference to this dispensation, respecting which it is said, “In the world ye shall have tribulation” (John 16:33)—the Lord spoke of a distinct period of distress, such as never had been, or should be again (Matt. 24:21-29). These verses refer to a great tribulation that shall fall upon the Jews in a future day: (compare Jer. 30:7 and Dan. 12:1). In Revelation 7:14 a great multitude is referred to that have come out of the great tribulation, but these are from the nations, hence this tribulation is not the same as that which will fall specially on the Jews, though both may take place at the same time. In Revelation 2:22 a “great tribulation” is spoken of, but it is doubtless general, and not the same as the above.

From Anstey’s Doctrinal Definitions:

The “great tribulation” (Matt. 24:21) refers to the terrible persecution that will be leveled at the faithful Jewish remnant during the latter half of the 70th week of Daniel (Dan. 9:27). It is also called “Jacob’s trouble” (Jer. 30:7), “the time of trouble” (Dan. 12:1), and “the hour of trial” (Rev. 3:10). It will last 1260 days (Rev. 11:3; 12:6).
The cause of this persecution will be a direct result of the faithful remnant of Jews refusing to take the mark of the Beast and to worship its image (Rev. 13:14-16). This will draw out the bitter hatred of the Beast and the Antichrist, and they will unleash the most terrible persecution against them that the world has ever known (Matt. 24:21). They will "make war with the saints" and will "overcome" many in martyrdom (Rev. 11:7; 12:13-17; 13:7, 15; Psa. 12:1; Isa. 57:1). The apostate Jews in the land, who will receive the Antichrist and worship the image of the Beast, will help in the persecution against their brethren. W. Scott mentions that this terrible period of persecution will be “shortened” by the Lord some 17½ or 18 days for the elect’s sake (Matt. 24:22). This is seen in the fact that the latter 3½ years of the prophetic week, which is 1278 days (42 months), will be reduced to 1260 days (Exposition of the Revelation of Jesus Christ, p. 172).
In the middle of the prophetic week, the personal Beast (“the little horn” – Dan. 7:8, 20-21) will rise up and take over the office of the Roman “prince” at the head of the western confederacy of ten nations—“the beast” (Rev. 13:1). (These western powers can also be referred to as the revived the Roman Empire.) Upon taking office, this new leader of the empire will breach the terms of the “covenant” (“the protection of abominations”) which will have been made with the unbelieving mass of the Jews (Dan. 9:27). The empire under this diabolical man will then seize control of the land of Israel and hold it as being part of the empire. He will abolish all Jewish worship to make way for a new kind of religion—“the abomination of desolation”—which is the worship of the beast and its image (Dan. 12:11; Matt. 24:15; Rev. 13:14-16). As a result, things will become very dark spiritually in the realm of the Beast—the West (Rev. 8:12). The true knowledge of God will be abolished in his realm and there will be a famine for the Word of God (Amos 8:11-12).
As mentioned, all in the realm of the Beast who refuse to receive the mark of the Beast and to worship the image will be persecuted. For many, it will be unto death (Rev. 13:7, 15). On account of the city of Jerusalem and the land of Israel being given up to the wickedness of idolatry that the Antichrist will bring in, and the persecution that will arise, the Jewish remnant will be forced to flee to the mountains, the caves, and the dens of the earth for safety. They will be hunted beyond measure (Psa. 42-72 – 2nd book of the Psalms; Matt. 24:16-21; Rev. 12:6, 14-15). The persecution will be so severe that people will hesitate to tell their thoughts to their spouses or members of their family for fear of being turned over to the authorities. A dearest friend will not be trusted (Jer. 9:4-5; Mic. 7:2, 5-6; Matt. 10:21-23).
Some of the Jewish remnant will flee into the mountains of Judea for shelter (Matt. 24:16). Others will flee eastward into the land of Moab—Jordan today (Isa 16:3-4; 1 Sam. 22:34; Psa. 44:11; Psa. 61:2). Others will go north to the Hermon mountain range in southern Lebanon (Psa. 42:6). Although most of the remnant will flee, a small number will remain behind in Jerusalem. God will use them to maintain an adequate testimony there for Himself (signified by "the two witnesses") in the face of the idolatry. They will be miraculously protected by God for the full duration of their testimony of 1260 days, and then He will allow them to be martyred (Rev. 11:3-13).
The Great Tribulation will be brought to a close by the attack of the King of the North and his Arab confederacy (Dan. 11:40-42). The apostate Jews who persecute the remnant will be slaughtered by the attacking armies. Their king (the Antichrist) who promised safety for the Jews will flee from his post in the land (Zech. 11:17) and will leave the bewildered apostate Jews who have trusted him to be slaughtered (Isa. 8:21-22). As many as 10 million apostate Jews will be killed by the attacking armies! (Zech. 13:8) After this, the Lord will appear from heaven to judge Israel’s enemies. He will restore and bless the nation (the surviving remnant of Jews and the real among the ten tribes) according to the promises of the Old Testament prophets (Rom. 11:26-27; Hos. 6:1-3; Mal. 4:2).

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