Why there will be great tribulation? What will be the cause or origin of the appearance of the great tribulation?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 anwer is in the Word of God, human opinions in accord the spirit of man which is in him is good for nothing.
The answer of the question is in Daniel 12:v.1 - 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: ... It is Michael, the same that was in Sodom, who is with his sharp Sword-God's Word- to pour severe plagues upon the nations ruled by men guided by the spirit of Devil, and they have already started in accord the Word of God. There will be severe punishments, even against powerful nations- Micah 4:v.3-, yeah, only and only punishments, consequently there will be great tribulation such as was not since the beginning of the world to this time, no, nor ever will be.
Be ready. The days will be shortened because there is a deadline to destroy the Devil's world (Revelation 11: v.18) and to close the Dispensation of Grace. He that will endure unto the end, the same will be saved.
Other great happening is determined for this present time. There will be resurrection in this present time, some to everlasting life, and some to shame and everlasting contempt.