The phrase "megas thlipsis" (great tribulation) appears 4 times in the New Testament. Each use of the phrase indicates a period of intense pressure (of some type), that had been experienced or was to be experienced by the objects of the “megas thlipsis.”
Stephen used the phrase to describe the effect of the famine, which precipitated the sons of Israel going to Egypt for food.
Act 7:11 Now there came a dearth over all the land of Egypt and Chanaan, and great affliction: and our fathers found no sustenance.
Our Lord Jesus used it as a warning of judgment upon the Jezebel in the church of Thyatira and those who were involved in adultery (presumed to be spiritual in nature, hence idolatry) with her unless they repent. This was a literal, first century, local church in the Roman province of Asia.
Rev 2:22 Behold, I will cast her into a bed, and them that commit adultery with her into great tribulation, except they repent of their deeds.
In another setting, "megas thlipsis" has the definite article before both megas and thlipsis. It appears in Greek, literally, word for word, "the tribulation, the great".
Rev 7:14 And I said unto him, Sir, thou knowest. And he said to me, These are they which came out of great tribulation, and have washed their robes, and made them white in the blood of the Lamb.
The dual use of the definite article signifies that it is a unique and recognizable event, one easily distinguished from other instances of "megas thlipsis". John is being told that the innumerable company of the redeemed, whom he saw gathered together, standing before the throne of God, came out of (or after) "the" great tribulation. So, there is a unique "megas thlipsis" that precedes the appearing of a great multitude of the redeemed ("which no man could number, of all nations, and kindreds, and people, and tongues") before the throne of God.
The last use of "megas thlipsis" and the one we will focus upon, comes in Christ's Olivet Discourse and in the context is said to be a time of severe persecution that will be unprecedented in nature. It is the unprecedented nature of this great tribulation, that gives us the certainty that it is "the tribulation, the great", from Rev 7:14.
Mat 24:21 For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
The language which indicates that the "megas thlipsis", that Christ says takes place before His parousia, is unprecedented, links it to that period of unprecedented tribulation (trouble) of the elect, which Daniel spoke of and Jeremiah prophesied about.
Dan 12:1 And at that time shall Michael stand up, the great prince which standeth for the children of thy people: and there shall be a time of trouble, such as never was since there was a nation even to that same time: and at that time thy people shall be delivered, every one that shall be found written in the book.
Jer 30:7 Alas! for that day is great, so that none is like it: it is even the time of Jacob's trouble; but he shall be saved out of it.
“Megas thlipsis”, in Matt 24:21, is referring to an unparalleled pressure in the form of persecution. The persecution is focused upon the elect of God. The use of "therefore" in verse 15 is identifying the initiating event of this "megas thlipsis" mentioned in verse 21, connecting it back to the "thlipsis" (persecution) upon the followers of Christ in verse 9. Verse 9, of Matt 24, speaks of believers (the elect) being hated, afflicted (thlipsis), and murdered for their identification with Christ.
Mat 24:9 Then shall they deliver you up to be afflicted, and shall kill you: and ye shall be hated of all nations for my name's sake.
In verse 15, Christ begins to provide further details of this acute eruption of intensely violent persecution (thlipsis) upon the elect, particularly its starting point. He says that when the abomination of desolation that was spoken of by Daniel the prophet takes place, that "then shall be great tribulation."
Mat 24:15-21 When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:) ... For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be.
The unique nature of this “megas thlipsis” ("such as was not since the beginning of the world to this time, no, nor ever shall be") is an indication to us that the use of “thlipsis” in Mat 24 does not refer to common or even less frequent and extreme tribulation, that is, the pressures or difficulties of life, no matter what their source may be. Neither can it refer to God's wrath and judgment upon unbelieving mankind in the day of the Lord. Mark records concerning “megas thlipsis” that: “For in those days shall be affliction [thlipsis], such as was not from the beginning of the creation which God created unto this time, neither shall be.” (Mar 13:19) Christ said there was none like it from the beginning of creation. 1656 years after the creation, the LORD destroyed the world with a great flood. If “megas thlipis” describes God’s wrath upon the unrighteous, how does the wrath of God in the great flood fit Christ’s statement concerning the future “megas thlipsis” being unprecedented in the history of mankind? It doesn’t.
The destruction of all but 8 souls in Noah's flood was just short of the total annihilation of mankind. There is no indication that the "day of the Lord," the future period of God's wrath upon the unbelieving Beast worshippers at the time of the end, will be more severe than the great flood was. Although it is true, that the day of the Lord will be a unique expression of the wrath of the Lamb, as John reveals in the trumpet and vial judgments, yet, it will come far short of the sort of destruction of life that took place in Noah’s flood. We know that many who remain upon the earth will enter into Christ's millennial kingdom, whereas, all those outside of the ark perished.
“Megas thlipsis”, in Mat. 24, refers to extreme pressure in the form of unprecedented violent persecution. It is not in the same category as an expression of the wrath of God, such as that which will take place in the day of the Lord or that which was witnessed by Noah. The "megas thlipsis" that Jesus spoke of, will involve an expression of Satan’s wrath upon the followers of Christ. John confirms, in Rev 13:7, what was revealed to Daniel, that a future king will afflict (wear out) the holy ones (saints) of the Most High.
Dan 7:25 And he shall speak great words against the most High, and shall wear out the saints of the most High, and think to change times and laws: and they shall be given into his hand until a time and times and the dividing of time.
This is a persecution of holy ones (saints), those made righteous in Christ. There is nothing holy about unbelievers, whether they be Jews or Gentiles. Satan’s man, the Beast (Rev 13:7), will be persecuting the followers of Christ (Jew and Gentile), to whom the blood of the Lamb had been applied (Rev 12:11).
Rev 12:7-17 And there was war in heaven … And the great dragon was cast out, that old serpent, called the Devil, and Satan, which deceiveth the whole world: he was cast out into the earth, and his angels were cast out with him. … the accuser of our brethren is cast down, which accused them before our God day and night. … And they overcame him by the blood of the Lamb, and by the word of their testimony; and they loved not their lives unto the death. Therefore rejoice, ye heavens, and ye that dwell in them. Woe to the inhabiters of the earth and of the sea! for the devil is come down unto you, having great wrath, because he knoweth that he hath but a short time. And when the dragon saw that he was cast unto the earth, he persecuted the woman which brought forth the man child. … And the dragon was wroth with the woman, and went to make war with the remnant of her seed, which keep the commandments of God, and have the testimony of Jesus Christ.
In conclusion, there are 4 uses of “megas thlipsis” in the NT, only two of them speak of the same event, a future period of the unprecedented persecution of the elect. Although persecution has been the lot of the righteous throughout the history of fallen mankind, at some point in the future (at the abomination of desolation) persecution shall increase in scope and intensity to an unprecedented level. This future unprecedented persecution of the righteous is THE GREAT TRIBULATION and it will be followed by the parousia of the Lord Jesus Christ.