A.T. Robertson Commentary 1 Thessalonians 4:15

"For this we say unto you by the word of the Lord, that we that are alive, that are left unto the coming of the Lord, shall in no wise precede them that are fallen asleep." — 1 Thessalonians 4:15 (ASV)

By the word of the Lord (εν λογω Κυριου). We do not know to what word of the Lord Jesus Paul refers, probably Paul meaning only the point in the teaching of Christ rather than a quotation. He may be claiming a direct revelation on this important matter as about the Lord's Supper in 1 Corinthians 11:23. Jesus may have spoken on this subject though it has not been preserved to us (cf. Mr 9:1).

Ye that are alive (ημεις ο ζωντες). Paul here includes himself, but this by no means shows that Paul knew that he would be alive at the Parousia of Christ. He was alive, not dead, when he wrote.

Shall in no wise precede (ου μη φθασωμεν). Second aorist active subjunctive of φθανω, to come before, to anticipate. This strong negative with ου μη (double negative) and the subjunctive is the regular idiom (Robertson, Grammar, p. 929). Hence there was no ground for uneasiness about the dead in Christ.