When Christ returns, we shall all be changed, including heaven and earth, actually all things are changed out for something better.

[Scott Downey]

1 Cor 15
50 Now this I say, brethren, that flesh and blood cannot inherit the kingdom of God; nor does corruption inherit incorruption.

51 Behold, I tell you a [m]mystery: We shall not all sleep, but we shall all be changed— 52 in a moment, in the twinkling of an eye, at the last trumpet. For the trumpet will sound, and the dead will be raised incorruptible, and we shall be changed.

53 For this corruptible must put on incorruption, and this mortal must put on immortality. 54 So when this corruptible has put on incorruption, and this mortal has put on immortality, then shall be brought to pass the saying that is written: “Death is swallowed up in victory.”

55 “O[n] Death, where is your sting?
O Hades, where is your victory?”
56 The sting of death is sin, and the strength of sin is the law. 57 But thanks be to God, who gives us the victory through our Lord Jesus Christ.

58 Therefore, my beloved brethren, be steadfast, immovable, always abounding in the work of the Lord, knowing that your labor is not in vain in the Lord.

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Some people think we shall not all be changed, some will remain flesh and blood people and have babies.
But that is not what scripture tells us here

The dead are raised.
We shall all be changed incorruptible.

We who remain alive until the Lord's coming are changed too, together with them that are dead in the Lord.
For this corruptible must put on incorruption, and this mortal must put on immortality.
The return of Christ is the Day of the Lord, unless you believe in a 3rd return of Christ, bur scripture mentions no such thing.
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Hebrews 9
27 And as it is appointed for men to die once, but after this the judgment, 28 so Christ was offered once to bear the sins of many. To those who eagerly wait for Him He will appear a second time, apart from sin, for salvation.
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The Day of Christ is the battle of that great day of God Almighty.

Paul says this

The Day of the Lord​

1 But concerning the times and the seasons, brethren, you have no need that I should write to you. 2 For you yourselves know perfectly that the day of the Lord so comes as a thief in the night. 3 For when they say, “Peace and safety!” then sudden destruction comes upon them, as labor pains upon a pregnant woman. And they shall not escape. 4 But you, brethren, are not in darkness, so that this Day should overtake you as a thief. 5 You are all sons of light and sons of the day. We are not of the night nor of darkness. 6 Therefore let us not sleep, as others do, but let us watch and be [a]sober. 7 For those who sleep, sleep at night, and those who get drunk are drunk at night. 8 But let us who are of the day be sober, putting on the breastplate of faith and love, and as a helmet the hope of salvation. 9 For God did not appoint us to wrath, but to obtain salvation through our Lord Jesus Christ, 10 who died for us, that whether we wake or sleep, we should live together with Him.

11 Therefore [b]comfort each other and [c]edify one another, just as you also are doing.

Peter says this

The Day of the Lord​

10 But the day of the Lord will come as a thief in the night, in which the heavens will pass away with a great noise, and the elements will melt with fervent heat; both the earth and the works that are in it will be [d]burned up. 11 Therefore, since all these things will be dissolved, what manner of persons ought you to be in holy conduct and godliness, 12 looking for and hastening the coming of the day of God, because of which the heavens will be dissolved, being on fire, and the elements will melt with fervent heat? 13 Nevertheless we, according to His promise, look for new heavens and a new earth in which righteousness dwells.

Paul says this

The Great Apostasy​

1 Now, brethren, concerning the coming of our Lord Jesus Christ and our gathering together to Him, we ask you, 2 not to be soon shaken in mind or troubled, either by spirit or by word or by letter, as if from us, as though the day of [a]Christ had come. 3 Let no one deceive you by any means; for that Day will not come unless the falling away comes first, and the man of [b]sin is revealed, the son of perdition, 4 who opposes and exalts himself above all that is called God or that is worshiped, so that he sits [c]as God in the temple of God, showing himself that he is God.

5 Do you not remember that when I was still with you I told you these things? 6 And now you know what is restraining, that he may be revealed in his own time. 7 For the [d]mystery of lawlessness is already at work; only [e]He who now restrains will do so until He is taken out of the way. 8 And then the lawless one will be revealed, whom the Lord will consume with the breath of His mouth and destroy with the brightness of His coming. 9 The coming of the lawless one is according to the working of Satan, with all power, signs, and lying wonders, 10 and with all unrighteous deception among those who perish, because they did not receive the love of the truth, that they might be saved. 11 And for this reason God will send them strong delusion, that they should believe the lie, 12 that they all may be condemned who did not believe the truth but had pleasure in unrighteousness.

Jesus says this

Sixth Bowl: Euphrates Dried Up​

12 Then the sixth angel poured out his bowl on the great river Euphrates, and its water was dried up, so that the way of the kings from the east might be prepared. 13 And I saw three unclean spirits like frogs coming out of the mouth of the dragon, out of the mouth of the beast, and out of the mouth of the false prophet. 14 For they are spirits of demons, performing signs, which go out to the kings [g]of the earth and of the whole world, to gather them to the battle of that great day of God Almighty.

15 “Behold, I am coming as a thief. Blessed is he who watches, and keeps his garments, lest he walk naked and they see his shame.”

16 And they gathered them together to the place called in Hebrew, [h]Armageddon.

 

[Scott Downey]

Jesus's teaching of His return is as a thief as He returns at a time when you do not expect, certainly the world is not expecting a sudden destruction. So then you be watching and wait on the Lord with expectation, especially when you see the signs. In other words, always be ready for His return.​

The Coming of the Son of Man​

29 “Immediately after the tribulation of those days the sun will be darkened, and the moon will not give its light; the stars will fall from heaven, and the powers of the heavens will be shaken. 30 Then the sign of the Son of Man will appear in heaven, and then all the tribes of the earth will mourn, and they will see the Son of Man coming on the clouds of heaven with power and great glory. 31 And He will send His angels with a great sound of a trumpet, and they will gather together His [d]elect from the four winds, from one end of heaven to the other.

The Parable of the Fig Tree​

32 “Now learn this parable from the fig tree: When its branch has already become tender and puts forth leaves, you know that summer is near. 33 So you also, when you see all these things, know that [e]it is near—at the doors! 34 Assuredly, I say to you, this generation will by no means pass away till all these things take place. 35 Heaven and earth will pass away, but My words will by no means pass away.

No One Knows the Day or Hour​

36 “But of that day and hour no one knows, not even the angels of [f]heaven, but My Father only. 37 But as the days of Noah were, so also will the coming of the Son of Man be. 38 For as in the days before the flood, they were eating and drinking, marrying and giving in marriage, until the day that Noah entered the ark, 39 and did not know until the flood came and took them all away, so also will the coming of the Son of Man be. 40 Then two men will be in the field: one will be taken and the other left. 41 Two women will be grinding at the mill: one will be taken and the other left. 42 Watch therefore, for you do not know what [g]hour your Lord is coming. 43 But know this, that if the master of the house had known what [h]hour the thief would come, he would have watched and not allowed his house to be broken into. 44 Therefore you also be ready, for the Son of Man is coming at an hour you do not expect.

The Faithful Servant and the Evil Servant​

45 “Who then is a faithful and wise servant, whom his master made ruler over his household, to give them food [i]in due season? 46 Blessed is that servant whom his master, when he comes, will find so doing. 47 Assuredly, I say to you that he will make him ruler over all his goods. 48 But if that evil servant says in his heart, ‘My master is delaying [j]his coming,’ 49 and begins to beat his fellow servants, and to eat and drink with the drunkards, 50 the master of that servant will come on a day when he is not looking for him and at an hour that he is not aware of, 51 and will cut him in two and appoint him his portion with the hypocrites. There shall be weeping and gnashing of teeth.