Then, in the next place, having either himself misunderstood the sacred Scriptures, or those (interpreters) by whom they were not understood, he proceeds to assert that “it is said by us that there will remain at the time of the visitation which is to come upon the world by the fire of purification, not only those who are then alive, but also those who are long ago dead;” not observing that it is with a secret kind of wisdom that it was said by the apostle of Jesus: “We shall not all sleep, but we shall all be changed, in a moment, in the twinkling of an eye, at the last trump; for the trumpet shall sound, and the dead shall be raised incorruptible, and we shall be changed.” Now he ought to have noticed what was the meaning of him who uttered these words, as being one who was by no means dead, who made a distinction between himself and those like him and the dead, and who said afterwards, “The dead shall be raised incorruptible,” and “we shall be changed.”
And as a proof that such was the apostle's meaning in writing those words which I have quoted from the first Epistle to the Corinthians, I will quote also from the first to the Thessalonians, in which Paul, as one who is alive and awake, and different from those who are asleep, speaks as follows: “For this we say unto you by the word of the Lord, that we who are alive and remain unto the coming of the Lord, shall not prevent them who are asleep; for the Lord Himself shall descend from heaven with a shout, with the voice of the archangel, and with the trump of God.” Then, again, after this, knowing that there were others dead in Christ besides himself and such as he, he subjoins the words, “The dead in Christ shall rise first; then we who are alive and remain shall be caught up together with them in the clouds, to meet the Lord in the air.”
Source: Against Celsus (New Advent)