“(Look! I will come like a thief! Blessed is the one who stays alert and does not lose his clothes so that he will not have to walk around naked and his shameful condition be seen.)” (Rev 16:15 NET)
Posttribulationists frequently use Revelation 16:15 to support their position. They argue that Rev 16:14–16 indicates that the second coming begins with the battle of Armageddon:
For they are the spirits of the demons performing signs who go out to the kings of the earth to bring them together for the battle that will take place on the great day of God, the All-Powerful. (Look ! I will come like a thief! Blessed is the one who stays alert and does not lose his clothes so that he will not have to walk around naked and his shameful condition be seen.) Now the spirits gathered the kings and their armies to the place that is called Armageddon in Hebrew.
There are good reasons, however, that this understanding of this passage is not correct. Just before the sixth bowl concludes there is an editorial exhortation: “(Look! I will come like a thief! Blessed is the one who stays alert and does not lose his clothes so that he will not have to walk around naked and his shameful condition be seen)” (Rev 16:15). Even in the midst of this apocalyptic portrayal there is a warning to be spiritually prepared to avoid God’s wrath. The book of Revelation warns us before (Rev 1:7), during (Rev 16:15), and after (Rev 22:12, 20) the narrative. This parenthetical exhortation in Rev 16:15 is not intended to give a temporal indicator to when the Lord will return, as is often interpreted. It does not mean that his parousia has not occurred up to this point with the church still on earth. Nor does it mean that his parousia will begin at Armageddon. It is a mistake therefore to read into the expression, “I come like a thief,” meaning that he has not returned yet. The parousia as outlined previously begins between the sixth seal and seventh seal. The trumpet judgments follow after the opening of the seventh seal, and the bowl judgments occur after the sounding of the seventh trumpet. This takes us up to the sixth bowl for the preparation of the battle of Armageddon. As noted earlier, it makes no sense to say that Jesus is coming back as a “thief” after the world just experienced his previous earth-destroying judgments. Translators such as the New English Translation cited above, recognize this verse as an aside, and thereby place parentheses around the statement. In a study note, the New English Translation comments: “These lines are parenthetical, forming an aside to the narrative. The speaker here is the Lord Jesus Christ himself rather than the narrator.”
So then how should we understand this parenthetical statement that seems to be arbitrarily placed at this point? The exhortation is immediately followed by the reference to the battle of Armageddon (v. 16). And since Armageddon occurs near the end of the culmination of God’s wrath, the likely meaning is that the warning represents the unified complex-whole of the day of the Lord’s wrath. This is consistent as we saw with Jesus and Paul’s use of the thief imagery representing the whole of God’s wrath (Matt 24:43–44; 1 Thess 5:2–5; cf. 2 Pet 3:10). In short, it is unintelligible to think that Rev 16:15 only intends to apply the thief warning to Armageddon but not to the previous trumpet and bowl judgments. When we speak of the return of Christ, it does not refer to an instantaneous simple event, instead, it encompasses the unified complex-whole of his return.
Next, the meaning of “I will come like a thief!” is followed by a qualification, “one who stays alert and does not lose his clothes so that he will not have to walk around naked and his shameful condition be seen” (cf. Rev 3:18). What is the implication of this statement? Some believe this refers to a believer who loses their salvation because they lose their faith and will be judged as an unbeliever along with the rest of the wicked. The question of whether believers can lose their salvation is not going to be determined by this single verse. My theological conviction is that Scripture consistently teaches the perseverance of the saints. I take this warning, as I do with other similar warnings in the Bible, as a means by which God perseveres his people by warning them to be spiritual vigilant. This warning is similar to Luke 21:36: “But stay alert at all times, praying that you may have strength to escape all these things that must happen, and to stand before the Son of Man.” And Matt 24:25: “Remember, I have told you ahead of time.” God’s sovereign grace will persevere believers, but it is our responsibility to be praying that we may have strength to escape all these things that must happen.
Rather, I take the warning to mean that the one who does not stay alert is revealed as a false believer, who will be found on the receiving end of God’s wrath. The thrust of the book of Revelation is on those who will have victory through enduring faith. It does not entertain lukewarm Christians as victorious, “So because you are lukewarm, and neither hot nor cold, I am going to vomit you out of my mouth!” (Rev 3:16). In the parable of the ten virgins, there are only two groups: wise and foolish virgins. All five wise virgins were spiritually alert when the bridegroom arrived (Matt 25:1–13). Jesus is going to refine his bride through the great tribulation so that he comes to a blameless people (Rev 14:12). The false believer is the one who will lose his clothes and walk around naked in a shameful condition. Being in a naked and shameful condition, therefore, indicates the person who will be exposed in the day of the Lord’s wrath. Faith is the true mark of a believer. But a person with a veneer of Christianity will find themselves thief-exposed in the midst of the Lord’s wrath.
In summary, this parenthetical warning within the apocalyptic day of God’s wrath teaches us that those who are not spiritually watchful for the Lord will find themselves experiencing these judgments. Hearing the book of Revelation read to them (Rev 1:3), John’s original audience would construe this warning—not as some clue to the timing for the parousia—but as a warning against idolatry, lest they experience the consequences of God’s wrath. But those who are spiritually watchful for the Lord’s return will find themselves fighting alongside the divine alliance: “[The kings of the world] will make war with the Lamb, but the Lamb will conquer them, because he is Lord of lords and King of kings, and those accompanying the Lamb are the called, chosen, and faithful” (Rev 17:14)
 Mounce, Revelation, 300–301.