"For then there will be great tribulation, such as has not been from the beginning of the world until now, no, and never will be." (Matthew 24:21)
Jesus here speaks of a time of great tribulation, one that is unparalleled in the history of the world. This great tribulation is one of the things which must happen before Jesus returns. The big question is this: Is this great tribulation past, present or future?
When Jesus spoke these words he spoke them to a certain generation, a generation (including his disciples) who would form the Church a short time later. It is possible that he spoke about this great tribulation for their benefit. If so, it would be equated with the several waves of severe persecution which hit the early church under the rule of the Roman empire. If that is so then the great tribulation has passed and there will never be a time like it again.
However... It could also speak of the whole church age. From the beginning the church has been persecuted including under the Roman empire. But it can be argued that there was worse to come. Since the fall of the Roman Empire until Napoleon the church has suffered a real and dangerous tribulation at the hands of the Roman Catholic church. Over the years many hundreds of thousands have been tortured and murdered for the sake of Christ.
Reading a book like "Foxe's book of Martyrs" or "The Pilgrim Church" by Mr EH Broadbent will give clear insight into the consistent persecution over the years. I am reminded of one story from Foxe's book about a blind boy named Thomas Drowry who was burnt to death on 15th May 1556:
"Dost thou not believe that after the words of consecration, spoken by the priest, there remaineth the very real body of Christ in the sacrament of that altar?"
"No," answered the blind boy, "that I do not."
"Then," said the Chancellor, "Thou art a heretic, and shalt be burned."
Dr Williams, then Chancellor of Gloucester, asked him who taught him such a thing, the boy answered that he the Chancellor had taught it:
"When you preached there" (naming a day) "a sermon to all men, as well as me, upon the sacrament. You said the sacrament was to be received spiritually, by faith, and not carnally and really, as the Papists have heretofore taught."
He was burned for his faith. Today, under communism or other strong regimes, people are still martyred for their faith.
Or the great tribulation could be in the future. All that has passed could be just a shadow of what is to come. John writes in his first letter, "Children, it is the last hour; and as you have heard that antichrist is coming..." (I John 2:18) and later in chapter 4, "...This is the spirit of antichrist, of which you heard that it was coming, and now it is in the world already."
The antichrist is coming, he has not yet come, his spirit, his influence is already in the world, "the prince of the power of the air, the spirit that is now at work in the sons of disobedience." But he himself has not yet come. When he comes, could it be that there will be a greater and worse tribulation than such as has not been from the beginning of the world?
Paul, by the Spirit, put it like this:
Let no one deceive you in any way; for that day will not come [i.e. the return of Christ - Gary], unless the rebellion comes first, and the man of lawlessness is revealed, the son of perdition, who opposes and exalts himself against every so-called god or object of worship, so that he takes his seat in the temple of God, proclaiming himself to be God. (2 Thessalonians 2:3-4)
In Mark 13 Jesus says "But in those days, after that tribulation..." This seems to link the tribulation, the other events mentioned there and ultimately the return of Christ to the same time period... "in those days..." Of course, it is not certain, time periods in prophet scripture can be very long.
But... we should watch and pray and be ready.