Paul points out that there are living (at the time of the letter) eyewitnesses to the resurrected Christ, himself included. “You don’t believe it? Go ask someone who was there!”
One of the interesting things Paul explains, in vs. 22-28, is there is an order to the resurrection of mankind. It was through the actions of Adam that all of mankind is subject to one day die. But it is through the actions of the second Adam, namely Jesus, that all shall be made alive. As Adam led the way for humanity in death, so Jesus leads the way for humanity in life.
23) But every man in his own order: Christ the firstfruits; afterward they that are Christ’s at his coming. (KJV)
The firstfruits are the first of the harvest which is offered up to God after the Passover lamb is slain. And so, when he died at Passover, Jesus became the firstfruits of those who were to be resurrected. When Jesus returns, meaning his Second Coming, Paul tells us here that those who belong to Christ will be resurrected. This fits with I Thess. 4-5 which explains that when the “Lord himself shall descend from heaven with a shout,” the “dead in Christ shall rise first.”
After these events, there will be an end when Jesus, after reigning on earth and putting all things under his feet, will deliver everything over to the Father by subjecting himself to the Father. And when Paul says everything will be put under Jesus’ feet, he means everything! Including death, which is the last enemy to be destroyed. (Check out Rev. 20:11-15 where death delivers up those who are dead, then death itself is cast into the lake of fire.)
What is the purpose for all this? Verse 28 explains it all.
28) And when all things shall be subdued unto him [Jesus], then shall the Son also himself be subject unto him that put all things under him, that God may be all in all.
This is our ultimate purpose. This is why Jesus died for us.