Albert Barnes Commentary 1 Thessalonians 5:2

"For yourselves know perfectly that the day of the Lord so cometh as a thief in the night." — 1 Thessalonians 5:2 (ASV)

For yourselves know perfectly. That is, they had been taught this. There could be no doubt in their minds concerning it.

The day of the Lord so cometh. This refers to the Lord Jesus—for the word "Lord" in the New Testament commonly means this (see Barnes on Acts 1:24).

The "day of the Lord" means that day in which he will be manifested, or in which he will be the prominent object in view of the assembled universe.

As a thief in the night. This happens suddenly and unexpectedly, as a robber breaks into a dwelling. A thief comes without giving any warning or any indications of his approach. He not only gives none, but he is careful that none shall be given. It is a point with him that, if possible, the man whose house he is about to rob shall have no means of ascertaining his approach until he comes suddenly upon him (Compare to Matthew 24:37 [see Barnes]; Matthew 24:38–43; Luke 12:39 [see Barnes]; and Luke 12:40 [see Barnes]).

In this way the Lord Jesus will return to judgment; and this proves that all attempts to determine the day, the year, or the century when he will come must be fallacious.

He intends that his coming to this world shall be sudden and unexpected, like that of a thief in the night; that there shall be no such indications of his approach that it shall not be sudden and unexpected; and that no warning of it is indeed the point of comparison in expressions like this. What else is it?

Is there anything else in which his coming will resemble that of a thief?

And if this be the true point of comparison, how can it be true that men can ascertain when that is to occur? Assuredly, if they can, his coming will not be like that of a thief (Compare to Acts 1:7 [see Barnes]).