The problem is that when people speak of the Rapture, they generally mean “The Secret Rapture”, which is a doctrine now taught in many denominations.  According to this doctrine, 

God will one day secretly take away from this earth all who are ready to go with Him to heaven.  The rest of the population will find that all the good Christians are gone, but they will all get a second chance to get ready for heaven, when God comes again.  This doctrine is not Biblical, but is built around the following text:

1 Thessalonians 5: 2   For you yourselves know perfectly that the day of the Lord so comes as a thief in the night.

The idea is that God will come quietly and stealthily and take away His people, but look at the following:

2 Peter 3: 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 burned up.

Revelation 3: 3   “Remember therefore how you have received and heard; hold fast and repent. Therefore if you will not watch, I will come upon you as a thief, and you will not know what hour I will come upon you.” 

According to the above passages, we can see that this will not be a quiet event, but the idea is of suddenness and unexpectedness, not quietness or secrecy.  Let’s look at some of the other texts about Christ’s second coming, and see whether they support a secret event:

Acts 1     ⁹ Now when He had spoken these things, while they watched, He was taken up, and a cloud received Him out of their sight. ¹⁰ And while they looked steadfastly toward heaven as He went up, behold, two men stood by them in white apparel, ¹¹ who also said, “Men of Galilee, why do you stand gazing up into heaven? This same Jesus, who was taken up from you into heaven, will so come in like manner as you saw Him go into heaven.”

Matthew 16: 27   For the Son of Man will come in the glory of His Father with His angels, and then He will reward each according to his works.

1 Thessalonians 4       ¹⁵ For this we say to you by the word of the Lord, that we who are alive and remain until the coming of the Lord will by no means precede those who are asleep. ¹⁶ For the Lord Himself will descend from heaven with a shout, with the voice of an archangel, and with the trumpet of God. And the dead in Christ will rise first. ¹⁷ Then we who are alive and remain shall be caught up together with them in the clouds to meet the Lord in the air. And thus we shall always be with the Lord.  

Revelation 1: 7   Behold, He is coming with clouds, and every eye will see Him, even they who pierced Him. And all the tribes of the earth will mourn because of Him. Even so, Amen.

2 Thessalonians 1     ⁷ and to give you who are troubled rest with us when the Lord Jesus is revealed from heaven with His mighty angels, ⁸ in flaming fire taking vengeance on those who do not know God, and on those who do not obey the gospel of our Lord Jesus Christ.

⁹ These shall be punished with everlasting destruction from the presence of the Lord and from the glory of His power, ¹⁰ when He comes, in that Day, to be glorified in His saints and to be admired among all those who believe, because our testimony among you was believed.

2 Thessalonians 2:8    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.

From the above it is clear that the Second Coming of Christ is not a secret, nor is it going to be quiet!   It will be a very glorious, visible, noisy and totally non-secret event, and “every eye will see Him”.  The great danger about the Secret Rapture doctrine is that it teaches that it doesn’t matter if you’re not ready, you’ll be given a second chance!  This is simply not Biblical and can lull many into a false sense of security. According to all the signs given in the Bible, Christ’s second coming is very near, and we will get only one chance.  The rapture is no secret, and don’t be fooled into believing it.  One final text:

1 Thessalonians 5     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. ⁶Therefore let us not sleep, as others do, but let us watch and be sober.

So, now that we’ve put away the rapture, and know that Jesus is coming a second time, not secretly but openly, with great glory and power, we can look at the 144,000. As I am sure you are aware, Revelation is a book that uses symbols, a LOT!  So when we look at anything in the book of Revelation, we have to first make sure whether this is symbology or literal language.  Let’s look at the passages where the 144,000 appear:

Revelation 7      ² Then I saw another angel ascending from the east, having the seal of the living God. And he cried with a loud voice to the four angels to whom it was granted to harm the earth and the sea, ³ saying, “Do not harm the earth, the sea, or the trees till we have sealed the servants of our God on their foreheads.” ⁴ And I heard the number of those who were sealed. One hundred and forty-four thousand of all the tribes of the children of Israel were sealed:

⁵ of the tribe of Judah twelve thousand were sealed;
of the tribe of Reuben twelve thousand were sealed;
of the tribe of Gad twelve thousand were sealed;
⁶ of the tribe of Asher twelve thousand were sealed;
of the tribe of Naphtali twelve thousand were sealed;
of the tribe of Manasseh twelve thousand were sealed;
⁷ of the tribe of Simeon twelve thousand were sealed;
of the tribe of Levi twelve thousand were sealed;
of the tribe of Issachar twelve thousand were sealed;
⁸ of the tribe of Zebulun twelve thousand were sealed;
of the tribe of Joseph twelve thousand were sealed;
of the tribe of Benjamin twelve thousand were sealed.

Revelation 14     Then I looked, and behold, a Lamb standing on Mount Zion, and with Him one hundred and forty-four thousand, having His Father’s name written on their foreheads. ² And I heard a voice from heaven, like the voice of many waters, and like the voice of loud thunder. And I heard the sound of harpists playing their harps. ³ They sang as it were a new song before the throne, before the four living creatures, and the elders; and no one could learn that song except the hundred and forty-four thousand who were redeemed from the earth. ⁴ These are the ones who were not defiled with women, for they are virgins. These are the ones who follow the Lamb wherever He goes. These were redeemed from among men, being firstfruits to God and to the Lamb. ⁵ And in their mouth was found no deceit, for they are without fault before the throne of God.

The first clue that something is not normal, is the list of the tribes of Israel.  In the other lists, the name of Joseph does not appear, but both of his sons, Ephraim and Manasseh do appear.  Also, the tribe of Dan does not appear in this list.  Additionally, Levi does not appear in the other lists, as God kept them separate from the other tribes to serve Him in the tabernacle and the temple, but does appear in this list.  So the fact that this is not at all the normal list of the tribes of the children of Israel, tells us that it is likely to be symbolic.

But there is more.  When the Assyrians took the ten northern tribes of Israel captive (Reuben, Gad, Asher, Dan, Ephraim, Manasseh, Issachar, Simeon, Naphtali and Zebulun), they were scattered among the nations and never returned to Palestine. All records of them disappeared.  Later the tribes of Judah and Benjamin (and also the Levites) were taken into captivity by the Babylonians, but after 70 years were allowed back to Palestine.  So, at present there is no record of the ten tribes which were lost, so how could there possibly be 12,000 of each of them sealed?

The final proof that we are not talking about the literal Jews here, but a symbolic group, is a number of texts that explain who the Israel is that is spoken about in the New Testament:

Romans 2     ²⁸ For he is not a Jew who is one outwardly, nor is circumcision that which is outward in the flesh; ²⁹ but he is a Jew who is one inwardly; and circumcision is that of the heart, in the Spirit, not in the letter; whose praise is not from men but from God.

Romans 10:12   For there is no distinction between Jew and Greek, for the same Lord over all is rich to all who call upon Him.

Galatians 3     ²⁸ There is neither Jew nor Greek, there is neither slave nor free, there is neither male nor female; for you are all one in Christ Jesus. ²⁹ And if you are Christ’s, then you are Abraham’s seed, and heirs according to the promise.

It therefore seems much more likely that the 144,000 of the children of Israel mentioned, is God’s people of all nations, tribes and tongues, and that the 144,000 is a symbolic number, signifying the entirety of God’s saved people.  This is an exclusive group, not because they are literal Jews, but because they are completely surrendered to God, following Him wherever He leads, and are therefore sealed in their foreheads, having God’s character in their minds.

So, to get to your question about the requirements for being in God’s kingdom forever, it is simply that we should know Him, have a close personal relationship with Him. Look at the passage below, where Jesus is talking to His Father.

John 17:3   And this is eternal life, that they may know You, the only true God, and Jesus Christ whom You have sent.

I trust that this is clear and has not confused you. To summarise, there is no such thing as the Secret Rapture, Jesus is coming in the same way that He left this earth, with clouds of angels, in power and great glory, to receive His children and take them back home to heaven with Him. There is no “second chance”, so we have to be ready now. And finally, the 144,000 is not a literal figure, but a symbol of the complete saved population of earth, from the time of Adam to the very end. And you can be part of them, by knowing God, and having a close personal relationship with Him.

God bless as you study this!