The Resurrected Christ is the Firstfruits

13 But we do not want you to be uninformed, brothers, about those who are asleep, that you may not grieve as others do who have no hope. 14 For since we believe that Jesus died and rose again, even so, through Jesus, God will bring with him those who have fallen asleep. 15 For this we declare to you by a word from the Lord, that we who are alive, who are left until the coming of the Lord, will not precede those who have fallen asleep. 16 For the Lord himself will descend from heaven with a cry of command, with the voice of an archangel, and with the sound of the trumpet of God. And the dead in Christ will rise first. 17 Then we who are alive, who are left, will be caught up together with them in the clouds to meet the Lord in the air, and so we will always be with the Lord. 18 Therefore encourage one another with these words. (1 Thessalonians 4:13-18 ESV)

The reality of this physical life in the temporal is that the death of our bodies is inevitable. This is the result of the fall of Man that we find in Genesis 3. This fall changed our race so that we are mortal instead of immortal. We age and eventually die. We can become ill and die. We can be killed in many different ways. We do not live in these bodies forever. Along with that part of the fall, Man also became spiritually dead. All are born as children of wrath because we have within us a Sin Nature. The presence of sin within us separates us from God who is Holy. Prior to the fall, Man was fulfilled in God because His presence with Man. Man was vertically or God oriented. After the fall that part of Man died. Now Man is horizontally or temporally oriented. Man seeks fulfillment in desperation through the flesh, but it is futile. Only God’s presence fulfills, but Man has no ability or desire to seek God and His presence. Tragically, all who are separate from God in this life will also be separate from Him when they die. They will die in their sins and be forever cutoff from Him, suffering His wrath for eternity.
However, God is merciful. Knowing that Man cannot do anything to solve the problem of separation from Him, He created the only way to salvation from an eternity in His wrath. The Son of God became a man. He was born of a virgin by the Holy Spirit, therefore, He had no inherent Sin Nature. He lived a sinless life in obedience to God the Father. When it was time, God caused the Son to be arrested, tortured, and executed as a criminal. As the Son went through this, God the Father imputed the sins of His people to Him. His wrath against that sin was poured out on the Son who was the perfect sacrifice, the Lamb of God, who paid the price for those sins thereby purchasing or redeeming those who receive Him as Lord and Saviour by faith through grace as they are regenerated. His perfect Righteousness is imputed to them. The Father then justifies them, declaring them Righteous. They are no longer in their sins.
Jesus Christ died on the cross bearing our sins. He was buried in a borrowed tomb, but did not stay there. His body never underwent decay. He was raised on the third day. He was resurrected. He is not a disembodied spirit hovering around, but has a real body that still bears the scars of His crucifixion.

20 But in fact Christ has been raised from the dead, the firstfruits of those who have fallen asleep. 21 For as by a man came death, by a man has come also the resurrection of the dead. 22 For as in Adam all die, so also in Christ shall all be made alive. 23 But each in his own order:Christ the firstfruits, then at his coming those who belong to Christ. 24 Then comes the end, when he delivers the kingdom to God the Father after destroying every rule and every authority and power. 25 For he must reign until he has put all his enemies under his feet. 26 The last enemy to be destroyed is death. 27 For “God has put all things in subjection under his feet.” But when it says, “all things are put in subjection,” it is plain that he is excepted who put all things in subjection under him. 28 When all things are subjected to him, then the Son himself will also be subjected to him who put all things in subjection under him, that God may be all in all. (1 Corinthians 15:20-28 ESV)

The Greek word translated as “firstfruits” here is απαρχη or aparche. It means, “the first of the ripe fruits.” This word is always singular. In this context (1 Corinthians 15) it is applied to Christ as the firstfruits of the resurrection with believers to follow later. By using this term this way, Paul is using an analogy of a harvest to portray the reality of eternal life in which our Lord’s resurrection happens first, guaranteeing that all of the saints who have died will also be resurrected.
The term “fallen asleep” is a common euphemism for death. There is no such thing in the Bible as “soul sleep” which refers to the body dying and the soul or spirit resting in unconsciousness. Adam, through his sin, brought death on the entire Human Race. Adam was Human as was Jesus Christ. However, Christ was resurrected after His death. This resurrection brought life to our race.
In v22 above we have this statement, “For as in Adam all die, so also in Christ shall all be made alive.” The first “all” refers to all of Adam’s descendants. All of them are born spiritually dead and are children of wrath. However, the second “all” refers to genuine believers only. It does not refer to all of Adam’s descendants. Some try to read into this passage universalism where all of Adam’s descendants will also be made alive in Christ. However, there are countless other passages in the Bible that teach eternal punishment of the unbelieving. (Matthew 5:29; Matthew 10:28; Matthew 25:41,46; Luke 16:23, 2 Thessalonians 1:9; Revelation 20:15)
When Christ returns it is the end of this age. It is at that time that he delivers the kingdom to God the Father. It will be the end of all rule and authority and power. All of that will be put under His feet. The last enemy, created at the fall, is death. Christ has broken the power of Satan. Satan held the power of death, but the cross was Christ’s victory over it and him. The only power not put under Christ’s feet is that of the Father who gave Christ His authority in the first place. Christ perfectly serves the Father always.
Christ is reigning right now over His kingdom which is in the hearts of His people. They have already been resurrected from death to life at their salvation, becoming New Creations. This Kingdom is alive and will last for eternity. However, the time is coming when He will bring an end to this age. His reign is eternal, but in the age to come, He will reign in His former, full and glorious place within the Trinity and those who follow Him into the harvest will serve Him forever.