The apostle Paul explains what happens when a believer dies: “But I do not want you to be ignorant, brethren, concerning those who have fallen asleep, lest you sorrow as others who have no hope. For if we believe that Jesus died and rose again, even so God will bring with Him those who sleep in Jesus” (1 Thessalonians 4:13-14 1 Thessalonians 4:13-14  But I would not have you to be ignorant, brothers, concerning them which are asleep, that you sorrow not, even as others which have no hope.
 For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.
American King James Version×). Here, we know the term "fallen asleep" is referring to death because he contrasts those who are asleep with "those who are alive and remain" (1 Thessalonians 4:15 1 Thessalonians 4:15For this we say to you by the word of the Lord, that we which are alive and remain to the coming of the Lord shall not prevent them which are asleep.
American King James Version×; compare Revelation 6:9-11 Revelation 6:9-11  And when he had opened the fifth seal, I saw under the altar the souls of them that were slain for the word of God, and for the testimony which they held:  And they cried with a loud voice, saying, How long, O Lord, holy and true, do you not judge and avenge our blood on them that dwell on the earth?  And white robes were given to every one of them; and it was said to them, that they should rest yet for a little season, until their fellow servants also and their brothers, that should be killed as they were, should be fulfilled.
American King James Version×). Then those in the faith who are still alive when Christ returns will be resurrected in the next instant after Christ returns (1 Thessalonians 4:17 1 Thessalonians 4:17Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
American King James Version×).
What is the kind of life that they will be resurrected to? It is life as a spirit being, rather than as a physical being (1 Corinthians 15:42-51 1 Corinthians 15:42-51  So also is the resurrection of the dead. It is sown in corruption; it is raised in incorruption:
 It is sown in dishonor; it is raised in glory: it is sown in weakness; it is raised in power:
 It is sown a natural body; it is raised a spiritual body. There is a natural body, and there is a spiritual body.
 And so it is written, The first man Adam was made a living soul; the last Adam was made a quickening spirit.
 However, that was not first which is spiritual, but that which is natural; and afterward that which is spiritual.
 The first man is of the earth, earthy; the second man is the Lord from heaven.
 As is the earthy, such are they also that are earthy: and as is the heavenly, such are they also that are heavenly.
 And as we have borne the image of the earthy, we shall also bear the image of the heavenly.
 Now this I say, brothers, that flesh and blood cannot inherit the kingdom of God; neither does corruption inherit incorruption.
 Behold, I show you a mystery; We shall not all sleep, but we shall all be changed,
American King James Version×; compare 1 Thessalonians 4:13-17 1 Thessalonians 4:13-17  But I would not have you to be ignorant, brothers, concerning them which are asleep, that you sorrow not, even as others which have no hope.  For if we believe that Jesus died and rose again, even so them also which sleep in Jesus will God bring with him.  For this we say to you by the word of the Lord, that we which are alive and remain to the coming of the Lord shall not prevent them which are asleep.  For the Lord himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God: and the dead in Christ shall rise first:  Then we which are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air: and so shall we ever be with the Lord.
American King James Version×, Revelation 11:15 Revelation 11:15And the seventh angel sounded; and there were great voices in heaven, saying, The kingdoms of this world are become the kingdoms of our Lord, and of his Christ; and he shall reign for ever and ever.
American King James Version×).
What about those who didn’t die “in Christ”? Are these people unsaved and condemned to burn forever in torment? No. For people not in this first resurrection, there is a second resurrection. It is described in Revelation: “I saw the souls of those who had been beheaded for their testimony about Jesus and for proclaiming the word of God . . . They all came to life again, and they reigned with Christ for a thousand years. This is the first resurrection. (The rest of the dead did not come back to life until the thousand years had ended.)” (Revelation 20:4-5 Revelation 20:4-5  And I saw thrones, and they sat on them, and judgment was given to them: and I saw the souls of them that were beheaded for the witness of Jesus, and for the word of God, and which had not worshipped the beast, neither his image, neither had received his mark on their foreheads, or in their hands; and they lived and reigned with Christ a thousand years.
 But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
American King James Version×, New Living Translation).
Notice the phrase “the rest of the dead.” Who are these people who are resurrected at the end of the thousand years? Quite simply, they are all those who are not in the first resurrection. So who is that? It is the vast majority of humanity that has ever lived, who never truly knew or understood who God is, or what his plan for man is. Most significantly, notice that the Bible says these people will live again!
Jesus Christ spoke about this second resurrection as well. In Matthew 12:38-42 Matthew 12:38-42  Then certain of the scribes and of the Pharisees answered, saying, Master, we would see a sign from you.
 But he answered and said to them, An evil and adulterous generation seeks after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas:
 For as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth.
 The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here.
 The queen of the south shall rise up in the judgment with this generation, and shall condemn it: for she came from the uttermost parts of the earth to hear the wisdom of Solomon; and, behold, a greater than Solomon is here.
American King James Version×, Christ condemns the religious leaders of His day, saying: “The men of Nineveh will rise up in the judgment with this generation and condemn it, because they repented at the preaching of Jonah; and indeed a greater than Jonah is here. The queen of the South will rise up in the judgment with this generation and condemn it, for she came from the ends of the earth to hear the wisdom of Solomon; and indeed a greater than Solomon is here.” Christ says these people will be resurrected (the word for "rise up" here is the same root word for resurrection we read earlier in Revelation 20:5 Revelation 20:5But the rest of the dead lived not again until the thousand years were finished. This is the first resurrection.
American King James Version×).
What an amazing concept! People who did not know Christ will indeed stand up once more, in a physical life, and have an opportunity to understand who God and Christ are and what their plan for all humanity is. The only way for this to be possible—for people who lived literally many hundreds of years apart to live again at the same time—is for God to resurrect all of them at the same time.
The Scriptures are plain about the fact that all people have only one chance at salvation. However, a second chance at life is not the same as a second chance at salvation. Coming to life again will enable them to have their only chance for salvation. To understand more about what happens after death, read What Happens After Death? and Heaven and Hell: What Does the Bible Really Teach?