Behold, I come as a thief - That is, suddenly and unexpectedly. See the Matthew 24:43 note; 1 Thessalonians 5:2 note. This is designed evidently to admonish people to watch, or to be in readiness for his coming, since, whenever it would occur, it would be at a time when people were not expecting him.
Blessed is he that watcheth - Compare Matthew 24:42-44. The meaning here is, that he who watches for these events, who marks the indications of their approach, and who is conscious of a preparation for them, is in a better and happier state of mind than he on whom they come suddenly and unexpectedly.
And keepeth his garments - The allusion here seems to be to one who, regardless of danger, or of the approach of an enemy, should lay aside his garments and lie down to sleep. Then the thief might come and take away his garments, leaving him naked. The essential idea, therefore, here, is the duty of vigilance. We are to be awake to duty and to danger; we are not to be found sleeping at our post; we are to be ready for death - ready for the coming of the Son of man.
Lest he walk naked - His raiment being carried away while he is asleep.
And they see his shame - Compare the notes on Revelation 3:18. The meaning here is, that, as Christians are clothed with the garments of righteousness, they should not lay them aside, so that their spiritual nakedness should be seen. They are to be always clothed with the robes of salvation; always ready for any event, however soon or suddenly it may come upon them.