What does Revelation 16:15 mean?

This verse is sometimes printed in parentheses, or in red letters, making it clear that it's a side note referring to the words of Jesus Christ. His statement presents a challenge to tribulation believers to stay alert. They will be blessed if they do so, knowing Jesus will return to earth suddenly and unexpectedly. His return will take unbelievers by surprise, just as a thief's visit takes a homeowner by surprise and is a frightful event. The apostle Paul informed the Thessalonian believers that the day of the Lord will come like a thief in the night (1 Thessalonians 5:2). The tribulation begins, therefore, like a thief in the night and also concludes like a thief in the night.

The allusion to keeping one's garments on, that he may not go about naked and exposed draws upon the custom of the day when men wore loose fitting robes. When a man walked quickly or ran or climbed stairs, the robe could become untied and fall open. Men wearing robes needed to be alert and careful about their behavior so their robes would not slip open. An unclothed person inside their house, also, is not prepared for a sudden visitor or to rush out in an emergency. Staying clothed, in a spiritual sense, means believers need to be careful about their behavior (Ephesians 5:8; 1 John 2:28).