I believe you are referring to what the Lord Jesus is saying in Matthew 24:21-24.

Let’s go back to the beginning of the Lord’s discourse here and read Matthew 24:4-8, “And Jesus answered and said unto them, Take heed that no man deceive you. For many shall come in my name, saying, I am Christ; and shall deceive many. And ye shall hear of wars and rumours of wars: see that ye be not troubled: for all these things must come to pass, but the end is not yet. For nation shall rise against nation, and kingdom against kingdom: and there shall be famines, and pestilences, and earthquakes, in divers places. All these are the beginning of sorrows.” 

Matthew, chapters 24-25 describe the seven-year Tribulation period and the judgment at the end of that period that will determine who will enter the Lord’s 1,000-year kingdom on the earth.  I encourage you to read both of these chapters to see the Lord’s judgment upon a world that has despised and rejected Him.  All the things described in these two chapters have to do with that time period, the world, and with the nation of Israel. 

With that in mind, let us read Matthew 24:21-24, “For then shall be great tribulation, such as was not since the beginning of the world to this time, no, nor ever shall be. And except those days should be shortened, there should no flesh be saved: but for the elect’s sake those days shall be shortened. Then if any man shall say unto you, Lo, here is Christ, or there; believe it not. For there shall arise false Christs, and false prophets, and shall shew great signs and wonders; insomuch that, if it were possible, they shall deceive the very elect.”

The Lord describes this time period as that of great tribulation…the worse the world has ever seen.  So much so, that if it continued, it would destroy every living creature in the world.  Who are these ‘elect’ ones mentioned here?  We read of this time in Daniel 12:1, “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: and at that time THY PEOPLE shall be delivered, every one that shall be found written in the book.”  This is in reference to the earthly people of God…the nation of Israel.  We also read in Isaiah 59:20, “And the Redeemer shall come to Zion, and unto them that turn from transgression in Jacob, saith the LORD.”  This speaks of the Lord’s return at the end of the Tribulation Period as He comes for those of those “that turn from transgression in Jacob.”  We can see that these “elect” are those Jewish believers living during the Tribulation Period.

I encourage you to read Romans, chapter 11 where Paul uses the analogy of the Olive tree to represent the cutting off of the Jews and the bringing in of the Gentiles.  We see the question put forth in verse 1, “Hath God cast away his people? God forbid.”  The earthly people of God, the Jews, have not been forgotten.  The Lord has not cast them away forever.  Verse 5 tells us that, “Even so then at this present time also there is a remnant according to the election of grace.”  Among the Jewish people, there still remains a remnant of those who truly believe on the Lord Jesus Christ.  At the moment, the nation of Israel has been “broken off” and the Gentiles now stand by faith. (Verses 19-20).  It is during the Tribulation Period that the remnant of believing Jews will be grafted back in as we see in verse 23, “And they also, if they abide not still in unbelief, shall be graffed in: for God is able to graff them in again.”  These are the “elect” that are mentioned in Matthew 24:21-24.  (436.6)