NOTES for Co1 15:51-58
As he concludes the discussion of transformation and resurrection, Paul draws a clear line between those who have chosen the Kingdom and those who have not (vv. 51-52). By "we" the apostle here undoubtedly means Christians, and by Christians he means the Church, which for him is nothing other than a community of people living in the Kingdom. It is no surprise that in this case Paul speaks not of death but of change or transformation: for those who have already chosen the Kingdom now, the last day will be something expected and more or less predictable, as it is for anyone who, after a long journey, has finally reached the intended and long-desired goal. Of course, the goal may turn out not to be exactly as those walking from far away imagined it, but in any case they will not be mistaken or disappointed in what matters most.
It is different for those who, for one reason or another, did not choose the Kingdom during their earthly journey. In itself this is not fatal, since a choice can be made even at the very last moment, and if it is sincere and the reasons for the earlier delay are weighty enough, the one who makes the choice will receive the Kingdom, like the workers of the eleventh hour in the Savior's parable. But for them the last day will be precisely the day of Judgment in the full sense of the word, because for them it will not be the completion of a path begun in advance, but the unexpected beginning of an entirely new life, and therefore one previously unknown to them. It is no accident that Paul speaks of them specifically in terms of resurrection, not of change or transformation: those who have become partakers of the Kingdom thereby already become, to a certain degree, partakers of the process that is completed at the end of time in the universal resurrection and transformation of human nature.
In a certain sense one could say that those who live in the Kingdom, even in the measure of fullness possible in our world, which has not yet been fully transformed, are already rising even now, though their resurrection will be completed at the end of time with the Savior's return. As for everyone else, for them everything will begin and end then, on the last day, though of course until the Last Judgment their destiny in eternity remains undefined. Only after each person's destiny has been determined and the process of universal transformation has been completed will the victory over death be complete (vv. 54-55). And that victory, in turn, will open the way to the fullness of that life and that breath of God with which the Kingdom is permeated.