This we say unto you by the word of the Lord - This I have, by express revelation, from the Lord: what he now delivers, he gives as coming immediately from the Spirit of God. Indeed, human reason could not have found out the points which he immediately subjoins; no conjectures could lead to them. Allowing even the general doctrine of the resurrection to be believed, yet what follows does not flow from the premises; they are doctrines of pure revelation, and such as never could have been found out by human ingenuity. In no place does the apostle speak more confidently and positively of his inspiration than here; and we should prepare ourselves to receive some momentous and interesting truth.

We which are alive, and remain - By the pronoun we the apostle does not intend himself, and the Thessalonians to whom he was then writing; he is speaking of the genuine Christians which shall be found on earth when Christ comes to judgment. From not considering the manner in which the apostle uses this word, some have been led to suppose that he imagined that the day of judgment would take place in that generation, and while he and the then believers at Thessalonica were in life. But it is impossible that a man, under so direct an influence of the Holy Spirit, should be permitted to make such a mistake: nay, no man in the exercise of his sober reason could have formed such an opinion; there was nothing to warrant the supposition; no premises from which it could be fairly deduced; nor indeed any thing in the circumstances of the Church, nor in the constitution of the world, that could have suggested a hint of the kind. The apostle is speaking of the thing indefinitely as to the time when it shall happen, but positively as to the Order that shall be then observed.

Shall not prevent them which are asleep - Those who shall be found living in that day, though they shall not pass through death, but be suddenly changed, shall not go to glory before them that are dead, for the dead in Christ shall rise first - they shall be raised, their bodies made glorious, and be caught up to meet the Lord, before the others shall be changed. And this appears to be the meaning of the apostle's words, μη φθασωμεν, which we translate shall not prevent; for, although this word prevent, from prae and venio, literally signifies to go before, yet we use it now in the sense of to hinder or obstruct. Φθανειν τινα signifies the same, according to Hesychius, as προηκειν, to go before, προλαμβανειν, to anticipate, be before. Those who shall be found alive on that day shall not anticipate glory before the dead in Christ; for they shall rise first, and begin the enjoyment of it before the others shall be changed. This appears to be the apostle's meaning.

by the.

1 Kings 13:1,9,17,18,22 And, behold, there came a man of God out of Judah by the word of the LORD to Bethel: and Jeroboam stood by the altar to burn incense...

1 Kings 20:35 And a certain man of the sons of the prophets said to his neighbor in the word of the LORD, Smite me, I pray you...

1 Kings 22:14 And Micaiah said, As the LORD lives, what the LORD said to me, that will I speak.

which are.

1 Corinthians 15:51-53 Behold, I show you a mystery; We shall not all sleep, but we shall all be changed...

2 Corinthians 4:14 Knowing that he which raised up the Lord Jesus shall raise up us also by Jesus, and shall present us with you.

prevent.

Job 41:11 Who has prevented me, that I should repay him? whatever is under the whole heaven is mine.

Psalm 88:13 But to you have I cried, O LORD; and in the morning shall my prayer prevent you.

Psalm 119:147,148 I prevented the dawning of the morning, and cried: I hoped in your word...

Matthew 17:25 He said, Yes. And when he was come into the house, Jesus prevented him, saying, What think you, Simon?...

asleep. See on ver.

1 Thessalonians 4:13 But I would not have you to be ignorant, brothers, concerning them which are asleep, that you sorrow not...

Chrysostom -- Excessive Grief at the Death of Friends
Chrysostom (that is, "Of the Golden Mouth") was a title given to John, Archbishop of Constantinople. He was born of a patrician family at Antioch about 347, and owed much to the early Christian training of his Christian mother, Anthusa. He studied under Libanius, and for a time practised law, but was converted and baptized in 368. He made a profound study of the Scriptures, the whole of which, it is said, he learned to repeat by heart. Like Basil and Gregory he began his religious life as a hermit …
Various—The World's Great Sermons, Volume I

Transformed by Grace
In the life of the disciple John true sanctification is exemplified. During the years of his close association with Christ, he was often warned and cautioned by the Saviour; and these reproofs he accepted. As the character of the Divine One was manifested to him, John saw his own deficiencies, and was humbled by the revelation. Day by day, in contrast with his own violent spirit, he beheld the tenderness and forbearance of Jesus, and heard His lessons of humility and patience. Day by day his heart …
Ellen Gould White—The Acts of the Apostles

Letter cxix. To Minervius and Alexander.
Minervius and Alexander two monks of Toulouse had written to Jerome asking him to explain for them a large number of passages in scripture. Jerome in his reply postpones most of these to a future time but deals with two in detail viz. (1) "we shall not all sleep but we shall all be changed," 1 Cor. xv. 51; and (2) "we shall be caught up in the clouds," 1 Thes. iv. 17. With regard to (1) Jerome prefers the reading "we shall all sleep but we shall not all be changed," and with regard to (2) he looks …
St. Jerome—The Principal Works of St. Jerome