Revelation 16:15

Jesus Christ Himself evidently gave this parenthetic invitation and warning (cf. Revelation 3:3; Revelation 3:18). His second coming will be as a thief in that it will be sudden, and His enemies will not expect it (cf. Matthew 24:43; Luke 12:39; 1 Thessalonians 5:2). Believers who understand the revelation of this book, on the other hand, will be expecting His return. Christ’s coming for the church will not be as a thief because the church is looking for His return (1 Thessalonians 5:4; Titus 2:13). Jesus Christ urged these faithful believers to be watchful and pure (cf. Matthew 25:1-30). [Note: Walvoord, The Revelation . . ., p. 238; Smith, A Revelation . . ., p. 235; Newell, p. 258.] The alternative is embarrassment (cf. Exodus 20:26; Leviticus 18:6-19; Deuteronomy 23:14; Isaiah 47:3; Ezekiel 16:37; Ezekiel 23:24-29; Hosea 2:10; Nahum 3:5). This is the third of the seven beatitudes in Revelation (cf. Revelation 1:3; Revelation 14:13; Revelation 19:9; Revelation 20:6; Revelation 22:7; Revelation 22:14).

Another interpretation sees this encouragement as directed to the Christian readers of this prophecy during the church age, before the Tribulation begins. Advocates of this view point out that by the sixth bowl believers who have not taken refuge (Revelation 12:13-17) will have suffered martyrdom (Revelation 13:15; Revelation 14:1-5; Revelation 14:13; Revelation 15:2). Therefore, according to this view, there will be no believers on the earth by the time the sixth bowl judgment occurs.

"The close similarity to Revelation 3:3; Revelation 3:18 and the parenthetical nature of the announcement favor the latter alternative [i.e., this view]." [Note: Thomas, Revelation 8-22, p. 267.]

I think this verse is a general word of encouragement addressed to believers in the Great Tribulation, in view of the context, but applicable to believers in the church age. If believers do not understand that Jesus Christ will return very soon, they may behave in ways that will be embarrassing when He does return, at the Rapture and the Second Coming.