A house of prayer

In the temple, animals were sold for the sacrifices, and money was changed for the temple tax. Jesus overturned the tables and quoted two prophets. Isaiah calls the temple a house of prayer. Mark quotes the whole verse, which adds that the house is meant for every nation.

The phrase ‘den of robbers’ comes from Jeremiah. A robbers' den is where bandits hide out after a raid. Jeremiah warned people who thought the temple would protect them, no matter how they lived. God's house is not a hideout for those who do wrong, but a place to meet God.