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Forty days has been the standard biblical period for trial, testing, and tribulation.
The pounding downpour and great upwelling of water during Noah’s Flood lasted 40 days, and even though they were preserved, surely Noah and all with him were greatly buffeted and distressed until God ended the storm.
At the beginning of the Exodus, 40 days marked the initial period of trial and testing of the Israelites. It ended only after they overcame the attack of Amalekites, after which they had rest.
Less than two years afterward, another 40-day test of Israel came in the searching out the land of Canaan by their twelve spies, and the Israelite’s response to their reports.
Elijah’s flight (while fasting) to Horeb, the Mountain of God, took 40 days.
Jesus, the Divine Human, underwent 40 days of trial (while fasting) at the beginning of his ministry.
Then later, after his resurrection, there came a concluding 40 days’ testing of the faith of the disciples up to the time of his ascension.
In our time, a sudden collapse of government, banking, the food distribution system and the electrical grid, coupled with chaos, war, and plague (the events of Seals 1-4), would bring near immediate tribulation for most people. Even those who are prepared might find it difficult to survive 40 days without either Divine or human (Church) assistance. This may be why the Lord said, “unless those days were [i.e., will be] shortened, no flesh would be saved.”
Paul clearly stated in Hebrews 12:18-28 and elsewhere that the Church’s coming to heavenly Mount Zion will mimic Israel’s coming to earthly Mount Sinai. I suspect this will apply to the time period also.
These are some of the reasons why I believe the Great Tribulation will last no longer than 40 days, and possibly some days less than that.
Despite the claims of some that this period will last 3½ or 7 years, not a single verse of Scripture actually says so. On the contrary, the 3½ times/1260-day period applies to earthly Israel, not to the Church. (See the blog post Revelation 12.)