https://youtu.be/u2tZh8LKzVY?si=7qL_VICaApVP8VAM The phrase den of thieves comes from Jeremiah 7 vs 11 were God says has this house, which bear's my name, become a den of robber's to you? The temple in Jerusalem become a den of thieves because people were using it for financial gain, turning the house of prayer into a market place for animal sacrifices and money changing. The temple courts were filled with merchant's selling animal for sacrifices and money changers who exchanged foreign currency for the required temple tax. The religious establishment including prophets and priests, were involved in this exploitative practices using the temple to cover up, to avoid punishment for their wicked deeds. People believed that the temple offered a secure refuge from God's judgement, even while disobeying his laws and committing injustices.
True worshippers were taken advantage through price gauging and deceptive practices which contradicted the temple's true purpose as a house of prayer. In the days of prophet Jeremiah God declared the temple a den of robber's because of this corruption, warning them that he will abandon it as he did with the Tabernacle of Shiloh if the people did not repent. Jesus act of chasing money changers and sellers was a direct message of Jeremiah's prophetic words, talking about the same hypocrisy and greed that had afflicted the temple earlier. They relied on their religion but did not live as God's people.