The article posted here explains how scripture holds the rapture being imminent, because Jesus said there is no set day or hour but to watch for the signs leading to the rapture as a separate event than his second coming, which is a separate topic than your question about imminency.
Imminent because it could happen at any time and the signs we are to watch for is all we have to go by.
Imminency, as it relates to Bible prophecy, simply means that the return of Jesus Christ for the Church can happen at any moment. No warning signs will indicate a short-term countdown. We as Christians remain on alert 24 hours a day, 7 days a week.
The only way for the rapture to be truly imminent is to have it transpire before the tribulation. If the Church were required to wait until after the manifestation of certain events, then there would be no doctrine of imminency.
The pretribulation rapture is the only view that allows for the rapture to be imminent in its timing. All the other views require a number of prophetic occurrences to take place before the rapture can be declared imminent. To be looking for the imminent return of Christ, you have to believe in a pre-trib rapture.
Jesus repeatedly said that His return for the Church would be a surprise. The Lord even went beyond that by saying He would return “as a thief” when believers generally won’t be expecting Him to come for them.
“But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only” (Matthew 24:36).”Watch therefore: for ye know not what hour your Lord doth come. But know this, that if the goodman of the house had known in what watch the thief would come, he would have watched, and would not have suffered his house to be broken up. Therefore be ye also ready: for in such an hour as ye think not the Son of man cometh. Who then is a faithful and wise servant, whom his lord hath made ruler over his household, to give them meat in due season? Blessed is that servant, whom his lord when he cometh shall find so doing” (Matthew 24:42-46 KJV).
“Watch therefore, for ye know neither the day nor the hour wherein the Son of man cometh” (Matthew 25:13).
“And he said unto them, It is not for you to know the times or the seasons, which the Father hath put in his own power”(Acts 1:7).
Because we have no way to refute the fact that we will not know the timing of our Lord’s return, the tribulation is a barrier to the rapture. No wonder the late John Walvoord called imminency “the heart of Pretribulationism.”
This type of “any-moment” language doesn’t fit a post-trib rapture. If Jesus were prevented from coming until after the battle of Magog, the rise of Antichrist, and the Mark of the Beast, we would have no need to watch for Him before the tribulation.
If the Church were required to go through the seven-year tribulation, you would expect the New Testament writers to have warned us to be prepared for trying times. On the contrary, the New Testament writers repeatedly tell the Church to be comforted by the “coming of the Lord” (1 Thes 4:18). The word “comfort” alone strongly implies the rapture will take place before the tribulation.
Some anti-imminency folks try to solve the problem they have with the rapture’s any-moment occurrence by redefining it as merely indicating that Christ will return soon. The speed of Christ’s advent is not the issue. If an event is required to take place before the Lord can return, there is no need to remain watchful.
If a person should make it through the tribulation until the point when the mid-trib, pre-wrath, and post-trib folks expect the rapture to occur, it would then become possible for the rapture to be classified as “imminent.” However, once you solve the problem of imminency, you create another one regarding the restrictions against knowing the timing of the rapture.
Because the duration of the tribulation is already known, post-tribbers have the hardest time dealing with the rapture’s timing. Some of them have tried to suggest that believers who make it through the tribulation will lazily lose track of the nearness of Christ’s second coming.
If a Christian has been lucky enough to survive a host of apocalyptic calamities and elude the Antichrist’s secret police for at least 3 1/2 years, I cannot imagine that he would be oblivious to the nearness of the Lord’s return at the 7-year mark. If I were reduced to the point of having to hide in a forest and forage through dead tree bark to find beetles and grubs to sustain myself, I’m certain my every thought would be focused on the Lord’s return.
Maranatha
One the strongest cases one can make for the early Church expecting an imminent return of Christ is to note their use of the word maranatha, which was used as a greeting in those days. When believers gathered or parted, they didn’t say “hello” or “goodbye”; they would say “Maranatha!”
I’ve encountered some writings that say Maranatha is Hebrew and Greek, but it is actually an Aramaic expression. In fact, it is made up of three Aramaic words: Mar, which means “Lord”; ana, which means “our”; andtha, which means “come.”
So when you put it together, maranatha means “Our Lord, come.” It perfectly conveys the concept that the Lord could come at any moment.Maranatha is used once in the Bible by Paul as part of a curse. In 1 Corinthians 16:22, Paul said, “If any man love not the Lord Jesus Christ, let him be Anathema. Maranatha.” The word “anathema” means “banned,” so Paul was saying, “Let him be banned from our Lord’s coming.”
The interesting thing about maranatha is that it comes in the form of a petition. When a Christian in the early Church would make this statement, he was actually petitioning the Lord to come. This obviously implies the belief that it was possible for Jesus to answer the appeal.
If members of the first-century Church believed that certain events needed to take place before the Savior could return, they would have been silly to greet each other with “maranatha.” They lived nearly 2,000 years ago, and yet they seem to have had a deeper awareness of imminency than many of today’s Christians.
But of that day and hour knoweth no man, no, not the angels of heaven, but my Father only” (Mat 24:36).Then shall the kingdom of heaven be likened unto ten virgins, which took their lamps, and went forth to meet the bridegroom. And five of them were wise, and five were foolish. They that were foolish took their lamps, and took no oil with them: But the wise took oil in their vessels with their lamps. While the bridegroom tarried, they all slumbered and slept. And at midnight there was a cry made, Behold, the bridegroom cometh; go ye out to meet him” (Mat 25:1-6)
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— Todd Strandberg
Complete article here:
https://www.raptureready.com/2016/07/19/imminency/
Reference as to the rapture and the second coming being two separate events is available upon request. Maranatha!