Despite our habit of trying to present ourselves in the best light, that will never be good enough for God. Our sin-stained bodies, the evidence of our rebellious nature, cannot enter God's presence. In the same way that we must be born again to be welcomed into God's kingdom, everything that reeks of sin must be replaced before we are welcomed into Christ's physical presence. We will be changed! Note, Paul does not say we will improve ourselves, but the change will be applied to us. God will give us a resurrection body and an uncontaminated heart.
 
Paul revealed a mystery. When the Lord Jesus comes again, a radical irreversible change will come upon born-again believers (both those who are alive at that time, and those whose bodies have long since disintegrated). That transformation will be instantaneous for all believers. The command of God will achieve it when the angelic trumpet call announces the final victory.
 
Until that day, death will remain the ultimate physical conqueror, naturally swallowing the best as well as the worst. But when Jesus returns, death itself will die, and will never be resuscitated. Isaiah puts it like this, "On this mountain he will destroy the shroud that enfolds all peoples, the sheet that covers all nations; he will swallow up death for ever. The Sovereign Lord will wipe away the tears from all faces; he will remove his people's disgrace from all the earth. The Lord has spoken." (Isaiah 25:7-8). As the shroud of death disappears, tears of sorrow and shame will also be gone for ever (Revelation 21:3-4).
 
Why does Paul tell us? It supplies essential hope to keep us trusting, obeying and waiting. Firstly, as believers who despair of our ability to improve ourselves, we need His promise that He will change us. Secondly, we need to understand that death does not have the last word, God does. Thirdly, as some wait in unbelievably difficult circumstances, we need to know that the anguish of this sinful world will not last for ever: eternity will contain no tears. Fourthly, as we all wrestle with bodies which refuse to obey us or God, we need to understand that death is not our release from sin and suffering: resurrection is (Philippians 3:12)! That is part of the gospel; and well worth proclaiming.