Know perfectly. — Or, accurately. There is something of an oxymoron (see Note on 1 Thessalonians 4:11) here. “I need not tell you about the time, for you know to a nicety — not the hour of Christ’s coming, but — the utter uncertainty respecting it.” The word shows at the same time with what scrupulous care St. Paul had instructed them on this topic.
The day of the Lord. — Here “the Lord” (as usual in the New Testament) means Jesus Christ; and this day can mean nothing else than the great day of His return to judgment. The expression is taken from the Old Testament, where, of course, it does not primarily mean what we call “the Day of Judgment,” but the set time which God has fixed for any great visitation. Thus in Joel 2:1, et seq., it means the time appointed for the plague of locusts; in Ezekiel 13:5, generally, any day when God visits His people; in Joel 3:14, the fixed time for vengeance to be taken upon the heathen for persecuting the Church; which, in Isaiah 2:12 (a passage largely influenced by recollections of Joel), seems to widen into a general day of judgment for mankind.
Cometh. — Not merely, will come; it is an absolute certainty that the time is on its way to come. (See Note on 1 Thessalonians 1:10.)
As a thief in the night — i.e. unexpectedly (Matthew 24:43), and under cover of darkness. The frequency of the simile (see references) throws light on the words “know perfectly,” making it apparent that it was the ordinary formula in which the doctrine was universally taught by the Apostles.