The text in the ninth chapter of the Apocalypse continues on to say that the rest, who weren’t slain by the judgment of the Sixth Trumpet, refused to repent of their murders, sorceries, fornications and thefts (Revelation 9:21). I don’t believe the sense has to be immediately following the deaths of one-third of the Jewish population. Rather, I think the sense pertains to during the judgment itself, while the deaths were occurring all around them, still the Jews didn’t consider their deeds and repent.
They had been persecuting the disciples of Jesus, killing some and bringing suffering upon others. This type of thing had been going on even beyond the borders of the Jewish state, as is testified in the epistles of James and Peter. The Jewish authorities had initiated a persecution against believers that extended up into Asia Minor and throughout the European churches that Paul had raised up.
As far as sorcery is concerned, first century Jewish theology seems to have been influenced by Persian dualism, picked up while the Jews were exiled to Babylon. Below, notice how the Jews’ Babylonian Talmud[1] depicts demonism, and consider the fact that Scripture is not used as their authority. Rather, the authority of “experts” is implied in the fact that the rabbis quoted one another (cf. Matthew 7:28-29):
It has been taught: Abba Benjamin says, If the eye had the power to see them, no creature could endure the demons. Abaye says: They are more numerous than we are and they surround us like the ridge round a field. R. Huna says: Every one among us has a thousand on his left hand and ten thousand on his right hand. Raba says: The crushing in the Kallah lectures comes from them. Fatigue in the knees comes from them. The wearing out of the clothes of the scholars is due to their rubbing against them. The bruising of the feet comes from them. If one wants to discover them, let him take sifted ashes and sprinkle around his bed, and in the morning he will see something like the footprints of a cock. If one wishes to see them, let him take the after-birth of a black she-cat, the offspring of a black she-cat, the first-born of a first-born, let him roast it in fire and grind it to powder, and then let him put some into his eye, and he will see them. Let him also pour it into an iron tube and seal it with an iron signet that they should not steal it from him. Let him also close his mouth, lest he come to harm. R. Bibi b. Abaye did so, saw them and came to harm. The scholars, however, prayed for him and he recovered. [Babylonians Talmud: Berachoth 6a; emphasis mine]
Such things are nothing more than sorcery. They have nothing to do with the word of God, nor is the logic based upon Scripture. Instead, these things seem to have their origin in magic and divination found in pagan chronicles.[2]
Whenever the Jews rebelled against God and followed the ways of others, the Scriptures refer to them as committing harlotry (cf. Jeremiah 2:20; 3:8-9). The Jewish state had rejected the Gospel preached by both Jesus and his disciples. They lived in the teachings of the dead (the elders) and they practiced whatever was right in their own eyes. They were, therefore, committing fornication and playing the harlot.
As for thefts, one could point out both literal thefts and spiritual. The Jewish authorities literally stole the tithes given the poorer priest, some of whom died, because they were dependent upon those offerings for their sustenance.[3] Spiritually, they were then presently involved in intimidating believers into leaving Jesus (stealing Jesus’ disciples) and returning to Judaism which nullifies Scripture (1Peter 1:6-7; 4:4, 12-13; 5:9).
It was things such as these that, although the Sixth Trumpet judgment had brought death to so many Jews, yet those who remained, continued in their murders, sorcery, thefts, and fornication, never even considering the evil they were doing was the thing that brought such horrendous judgment upon them. Their hearts were hardened and they simply would not repent.
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[1] Although compiled over the next few centuries, the theological perspectives of the Babylonian Talmud reflect that of first century Jews.
[2] See the Apocrypha for what an angel tells a young man to do with the heart, liver and gall of a fish (Tobit 6:1-8). Tobit is a work compiled several centuries before Christ.
[3] Josephus: Antiquities of the Jews 20.9.2