What does Revelation 9:21 mean?

This verse informs us that unbelievers who are not killed by the plagues of this chapter refuse to repent of their evil deeds. This follows a pattern clearly established in the Bible: human beings, in general, are stubbornly disobedient and sinful. Even when facing consequences, the natural human response is to cling to sin and selfishness.

In the tribulation, crimes such as murder will rise to unprecedented figures. As Jesus predicted, "lawlessness will be increased" in the tribulation (Matthew 24:12).

The unrepentant also refused to abandon their "sorceries." This is an interesting term, and one easily misunderstood in the modern world. For those raised in modern western culture, the term "sorcerer" evokes the idea of a magic-user or a spell-caster. There is a sense in which that kind of magic—or attempts at it—are part of man's rebellion against God. When Paul preached at Ephesus, converts to the gospel burned their books of magic arts worth fifty thousand pieces of silver (Acts 19:19). However, the Greek root word translated here as "sorceries" is pharmakeia. We get the modern English words "pharmacy" and "pharmaceuticals" from this term. As such, this is a reference to drugs, poisons, potions, and other illicit substances. Ancient "sorcerers" were those who controlled the minds or actions of others, often by use of such chemicals. It's reasonable to think drug use, abuse, and trafficking will reach huge proportions in the tribulation.

Sexual immorality will also abound in the tribulation. Despite all signs that God is judging humanity for their sin, those who indulge their sexual passions in illicit ways refuse to abandon their vile habits. Nor will they stop stealing and robbing. The unwillingness of these offenders to repent illustrates the desperate wickedness of the human heart (Jeremiah 17:9). It speaks to the justice of God's wrath on earth.