In Isaiah 23:17-18, why is the profit of a prostitute set apart for the LORD? If wicked persons' income is spent in the service of the LORD, won't the wicked be inspired to sin even more? Would it be fair to say that this is an example of God knowing that a people will not repent, allowing them to exercise their free will, but preventing them from benefiting/profiting from their sin?

Let me share a number of thoughts about Tyre and the money. (Be sure to read the passage first, if you are unfamiliar with it.)

  • The passage doesn't say the Tyrians would be aware that their profits, once frittered away, would flow southwards to their enemy Israel. So I doubt they'd be emboldened to sin.
  • The Tyrians didn’t repent, and God did indeed use the situation for his own purposes.
  • Tyre did rebuild a few decades later, though Alexander the Great took them out in 332 BC, early in his military career. That was the Tyrians' final end. I have visited the archaeological ruins of Tyre and it’s still there, but any prophecies about it have long been fulfilled. The place has no biblical significance for the modern Middle East.
  • The earlier fall of Tyre (referred to in the Isaiah passage you cited) comes at the end of the Assyrian conquest. In a way the fall of this important city-state is the conclusion of the conquest of Joshua. We recall that the Phoenicians who settled Canaan were also in Tyre, which is a Phoenician stronghold. So judgment would come.
  • The wages were to end up somehow supporting God’s temple in Jerusalem. There is no record of which I am aware of Tyrian money flowing directly into the coffers of Jerusalem. Yet I find it interesting that one of the coins in my study, which I purchased in Israel, is a Tyrian shekel. This was used for paying temple tax (for two people [as in Matt 17] — it is a tetradrachma). And the coins with which Judas Iscariot was paid were likely Tyrian shekels. Interesting, eh?