"For then there will be great distress, unequaled from the beginning of the world until now -- and never to be equaled again. If those days had not been cut short, no one would survive, but for the sake of the elect those days will be shortened" (Matthew 24:21-22).
The Tribulation is a name many use to refer to the time of "great distress" spoken of by Jesus in Matthew 24. It is viewed by some as being identical to the seven-year period spoken of by the prophet Daniel: "He will confirm a covenant with many for one 'seven.'" (Daniel 9:27). Others view this period as being, not seven years, but an as yet undetermined period of time.
Not all Christians accept the Tribulation as a future event. Some believe these prophecies were fulfilled in the first century, while others accept them as symbolic references to the 'tribulation' experienced by Christians in all ages.
Those who do accept the Tribulation as a future event believe that it will begin with the Antichrist's signing of a treaty with Israel, and will end with Christ's second coming at the battle of Armageddon. In this view, the first half of the Tribulation is generally viewed as a time of peace, when the Antichrist solidifies his rule over the nations. At the midpoint he performs the 'abomination of desolation' spoken of by Daniel: "In the middle of the 'seven' he will put an end to sacrifice and offering. And on a wing of the temple he will set up an abomination that causes desolation" (Daniel 9:27). It is during the second half of the Tribulation that the Antichrist's kingdom collapses, and God's judgments are poured out upon a Christ-rejecting world.
The exact nature of the tribulation is an interesting topic for discussion, but it is not an issue over which to break fellowship.