The Bible defines the Great Tribulation as a period of immense suffering, great persecution, and severe trial for all those who will refuse to worship the beast (the Antichrist) and his image.

Revelation 13:15
"And he had power to give life unto the image of the beast, that the image of the beast should both speak, and cause that as many as would not worship the image of the beast should be killed."

According to the Bible, it will be the Antichrist — also called the beast, the horn, or the lawless one — who will be the author and instigator of the Great Tribulation.

Revelation 13:1, 7
"¹And I stood upon the sand of the sea, and saw a beast rise up out of the sea, having seven heads and ten horns, and upon his horns ten crowns, and upon his heads the name of blasphemy. [...] ⁷And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations."

Daniel 7:21
"I beheld, and the same horn made war with the saints, and prevailed against them."

Revelation 13:7
"And it was given unto him to make war with the saints, and to overcome them: and power was given him over all kindreds, and tongues, and nations."

Daniel 7:21
"I beheld, and the same horn made war with the saints, and prevailed against them."

Through these passages, the Bible teaches us that it is the Saints who will endure the Great Tribulation. But who, according to the Bible, are these Saints? What does the Bible define as a Saint?

1 Corinthians 1:1–3
"¹Paul, called to be an apostle of Jesus Christ through the will of God, and Sosthenes our brother, ²unto the church of God which is at Corinth, to them that are sanctified in Christ Jesus, called to be saints, with all that in every place call upon the name of Jesus Christ our Lord, both theirs and ours: ³grace be unto you, and peace, from God our Father, and from the Lord Jesus Christ."

Philippians 1:1–2
"¹Paul and Timotheus, the servants of Jesus Christ, to all the saints in Christ Jesus which are at Philippi, with the bishops and deacons: ²grace be unto you, and peace, from God our Father, and from the Lord Jesus Christ."

Revelation 19:7–8
"⁷Let us be glad and rejoice, and give honour to him: for the marriage of the Lamb is come, and his wife hath made herself ready. ⁸And to her was granted that she should be arrayed in fine linen, clean and white: for the fine linen is the righteousness of saints."

We understand from these texts that a Saint is none other than a person who believes in the redemptive work of Christ on the cross, and who is thereby set apart (sanctified) to serve Him (1 Thess. 1:9). We can therefore readily see that the Saints spoken of in Revelation 13 and Daniel 7 are none other than the Church — that is, all those who have faith in Jesus Christ. It is not without reason, then, that the Lord exhorts these Saints to hold fast to their faith in Jesus Christ during this period of Great Tribulation.

Revelation 14:12
"Here is the patience of the saints: here are they that keep the commandments of God, and the faith of Jesus."

God will permit the Church to pass through the Great Tribulation for several reasons:

• The Great Tribulation will enable the Church to serve as a witness to all nations through the great sufferings she will endure.

  Mark 13:9

  "But take heed to yourselves: for they shall deliver you up to councils; and in the synagogues ye shall be beaten: and ye shall be brought before rulers and kings for my sake, for a testimony against them."

  Luke 21:12–13

  "¹²But before all these, they shall lay their hands on you, and persecute you, delivering you up to the synagogues, and into prisons, being brought before kings and rulers for my name's sake. ¹³And it shall turn to you for a testimony."

• It is during the Great Tribulation that the Church will demonstrate to the powers of this world — that is, to the principalities and authorities in the heavenly places — the manifold wisdom of God; the same wisdom that Jesus Christ, the Head of the Church, had already demonstrated to them by allowing Himself to be nailed to the cross by them (1 Cor. 2:6–8).

• It is during the Great Tribulation that God will refine, purify, and make white His children.
  

  Daniel 11:35

  "And some of them of understanding shall fall, to try them, and to purge, and to make them white, even to the time of the end: because it is yet for a time appointed."

• It is only through the Great Tribulation that the Church will be able to fully conform to the sufferings of Christ.
  

  Romans 8:17
  "And if children, then heirs; heirs of God, and joint-heirs with Christ; if so be that we suffer with him, that we may be also glorified together."

• It is only through the Great Tribulation that the Church will be able to attain perfection — and thus appear holy, without spot or wrinkle, before the Lord Jesus Christ on the day of His coming (Ephesians 5:27).

  1 Peter 4:1–2

  "¹Forasmuch then as Christ hath suffered for us in the flesh, arm yourselves likewise with the same mind: for he that hath suffered in the flesh hath ceased from sin; ²that he no longer should live the rest of his time in the flesh to the lusts of men, but to the will of God."

• It is during the Great Tribulation that God will save a great multitude of people.

  Revelation 7:9–14
  "After this I beheld, and, lo, a great multitude, which no man could number, of all nations, and kindreds, and people, and tongues, stood before the throne, and before the Lamb, clothed with white robes, and palms in their hands; And cried with a loud voice, saying, Salvation to our God which sitteth upon the throne, and unto the Lamb. And all the angels stood round about the throne, and about the elders and the four beasts, and fell before the throne on their faces, and worshipped God, Saying, Amen: Blessing, and glory, and wisdom, and thanksgiving, and honour, and power, and might, be unto our God for ever and ever. Amen. And one of the elders answered, saying unto me, What are these which are arrayed in white robes? and whence came they? 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 Bible describes a specific event that will trigger the Great Tribulation. That event is the abomination of desolation. What, then, is the abomination of desolation? The abomination of desolation refers to the act by which the Antichrist will seat himself in the Temple of God (this will be the third Temple of Jerusalem) and proclaim himself to be god (2 Thessalonians 2:1–4).

Matthew 24:15–22
"¹⁵When ye therefore shall see the abomination of desolation, spoken of by Daniel the prophet, stand in the holy place, (whoso readeth, let him understand:) ¹⁶then let them which be in Judaea flee into the mountains: ¹⁷let him which is on the housetop not come down to take any thing out of his house: ¹⁸neither let him which is in the field return back to take his clothes. ¹⁹And woe unto them that are with child, and to them that give suck in those days! ²⁰But pray ye that your flight be not in the winter, neither on the sabbath day: ²¹for then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. ²²And except those days should be shortened, there should no flesh be saved: but for the elect's sake those days shall be shortened."

According to the Bible, the Great Tribulation will last 1,290 days — that is, 3.5 years, or 42 months. This duration is also described as "a time, times, and half a time": one time being equal to one year, times to two years, and half a time to half a year.

Daniel 12:11
"And from the time that the daily sacrifice shall be taken away, and the abomination that maketh desolate set up, there shall be a thousand two hundred and ninety days."

Daniel 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."

Why must it last 3.5 years? The number 3.5 is half of 7 — 7 representing that which is complete or perfect, and 3.5 representing that which is incomplete or imperfect. If the tribulation lasts 3.5 years, it is to signify that the persecution and suffering will not be total; for if it were 7 years — that is, complete — no one would be saved. This is precisely why the Lord has shortened those days.

Mark 13:20
"And except that the Lord had shortened those days, no flesh should be saved: but for the elect's sake, whom he hath chosen, he hath shortened the days."

Revelation 15:1
"And I saw another sign in heaven, great and marvellous, seven angels having the seven last plagues; for in them is filled up the wrath of God."

What does this first verse of Revelation 15 tell us? It speaks of seven angels holding seven plagues — and it informs us, first of all, that it is through these seven plagues or vials that the wrath of God is manifested and fulfilled. Second, we all know that during the Great Tribulation, many people — especially Christians — will be killed: beheaded or burned alive, because of the Word of God or their refusal to worship the beast and his image. If the Great Tribulation were the wrath of God, then in what sense could being beheaded for the sake of God or His Word be considered a plague or a vial? In what sense could being burned alive for the name of God be considered a plague or a vial? The Great Tribulation — during which many will be slain by the Antichrist — is distinct and separate from the wrath of God, which will be manifested through the seven plagues.

Furthermore, we know that the outpouring of God's wrath through the seven plagues is described in chapter 16 of the book of Revelation; and nowhere in that description does the Bible attribute the beheading of Christians or saints by the Antichrist to any one of those plagues. The Great Tribulation, therefore, is not the wrath of God.

May the Lord bless you!