Bible Commentary
51 Paul uses the word "sleep" as a metaphor for death (John 11:11). He calls attention to the fact that there are some who will not die, but who will be translated from the imperfect physical state to the perfect heavenly state. With regard to both those alive when Jesus comes and those who have died, the former instantaneously exchange their mortal bodies for immortal bodies; the latter are raised with immortal bodies.²
52 The change in the bodies of the saints will occur in an instant.^2,3 The time when this glorious transformation takes place will be at the second coming of Christ, for it is then that the "trump of God" will sound, and faithful believers who have died will be raised in bodies that are entirely free from all effects of sin. Then Christians who are alive and looking eagerly for the coming of their Lord will undergo a marvellous change, whereby all traces of corruption and imperfection will be removed from their bodies, which will be made like unto Christ's glorious body (see Phil. 3:20, 21; 1 John 3:2).²
53 It is essential that a change take place in the bodies of the saints. And this will take place either by their dying and being raised from the dead in immortal, incorruptible bodies, or by their being changed to that state without seeing death. Although the body is changed, the redeemed will retain their individual personal identity or character. The gift of immortality will be received only by those who accept God's offer of salvation through Jesus Christ, and this gift will be theirs when Jesus comes again.²
54 When, at Christ's coming, the amazing transformation from mortal to immortal has taken place, both of the righteous dead and the righteous living, then man's great enemy will no longer trouble the redeemed. The last thought that occupied their minds as the shadow of death overtook the saints was that of approaching sleep, their last feeling was that of the pain of death. As they see that Christ has come and conferred on them the gift of immortality, their first sensation will be one of great rejoicing that never again will they succumb to the power of death.²
55 In the glad and victorious cry "O death, where is thy sting? O grave, where is thy victory?" both death and the grave are personified and addressed, probably by the saints, who will be delivered forever from the threat of the suffering and separation caused by death. The sway that this enemy has held over all men ever since the fall of Adam will be forever removed from the redeemed at the second coming of Christ.²
56 The "sting of death" is here defined as "sin". Death, like a scorpion, has a sting, a fatal power imparted to it by means of sin, the cause of death (Romans 6:23). But the redeemed will never again commit sin: therefore they can never again feel the sting of death ((Nahum 1:9; Revelation 21:4).²
57 This verse presents the theme of all Scripture, namely, to show that the restoration of man to favor with God and to his original condition of perfection and freedom from all the effects of sin, is brought about by the mighty power of God working through our Lord Jesus Christ. For this triumph over the power of the adversary the redeemed will give praise and glory to God throughout eternity.²

References and notes
1.  King James Authorized Version
2.  Seventh-day Adventist Bible Commentary Vol. 6 pgs 655, 812-813
3.  Adam Clarke's Commentaries - http://www.godrules.net/library/clarke/clarke.htm
4.  CATHOLIC ENCYCLOPEDIA: Epistles to the Corinthians - http://www.newadvent.org/cathen/04364a.htm
5.  Christian Resource Centre (Bermuda) Horn, Siegfried - http://www.nisbett.com/summary/sum-n-07.htm
6.   People's New Testament -
 http://bible.crosswalk.com/Commentaries/PeoplesNewTestament/pnt.cgi?book=1co&chapter=015