Jesus explains what He has just done by pointing to God's own word. "Is it not written" assumes His hearers already knew Isaiah 56:7, where God calls His temple "an house of prayer for all people." God meant His house for prayer, and for all nations.

Gill explains that not only Jews came to pray, but also Gentiles "who became of the Jewish religion, and had a court built for that purpose." Barnes says the Court of the Gentiles was "the place occupied by the buyers, and sellers, and the money-changers." The trade filled the very space where outsiders could come near to pray.

Then Jesus names what it had become: "a den of thieves." Gill says the buyers and sellers were no better than thieves in the Lord's eyes, along with "the priests that encouraged them, and had a profit out of them."

Mark records no answer from the sellers or the crowd. Instead, "the scribes and chief priests heard it, and sought how they might destroy him: for they feared him, because all the people was astonished at his doctrine" (Mark 11:18). When evening came, Jesus went out of the city (Mark 11:19). The next day the chief priests, the scribes, and the elders came to Him in the temple and asked, "By what authority doest thou these things?" (Mark 11:27-28).

Jesus quotes Isaiah 56:7. In Isaiah, the promise is for "the sons of the stranger, that join themselves to the LORD": God would bring them to His holy mountain and make them "joyful in my house of prayer" (Isaiah 56:6-7). Of the three Gospels that record this saying, only Mark keeps the words "of all nations" (compare Matthew 21:13; Luke 19:46).

"A den of thieves" echoes Jeremiah 7:11, where the LORD asks, "Is this house, which is called by my name, become a den of robbers in your eyes?" Gill says Jesus was "affirming what is complained of" in Jeremiah 7:11 "and applying it to the present case."

The trade itself served worship. Barnes notes that "Doves were required to be offered in sacrifice" and that every Jewish man paid a half shekel to the temple, which required changing money. But Barnes says, "They made the temple a place of gain; they cheated and defrauded; they took advantage of the poor." Mark adds that Jesus would not let anyone carry a vessel through the temple (Mark 11:16), which Gill explains as using it as a thoroughfare, a shortcut for common goods.

This question reveals God's heart for His house and for the nations. God intended His temple to welcome outsiders who turned to Him (Isaiah 56:6-7). Matthew Henry notes that when Solomon dedicated the temple, "it was with an eye to the sons of the strangers." The busy trade in the temple courts had turned that welcome into a marketplace.

Matthew Henry also writes that Christ began to reform the temple courts "to show that when the Redeemer came to Zion, it was to turn away ungodliness from Jacob." Jesus came to God's house and restored its purpose. Soon after, Mark records Jesus teaching His disciples about prayer and forgiveness: "when ye stand praying, forgive" (Mark 11:25).