What does Matthew 12:21 mean?

This concludes Matthew quotation of Isaiah 42:1–4. In that passage, Isaiah quotes God's description of the servant He loves and is pleased with. That servant will be empowered by the Holy Spirit to proclaim justice to the nations, which in this context means the Gentiles. Matthew is revealing Isaiah's passage to be about the Messiah, who is Jesus (Matthew 12:18–20).

This is not the Messiah Israel is expecting, however. Instead of confronting and overwhelming Israel's enemies, this Messiah will refuse to quarrel or cry out. Instead of breaking the Roman army, this Messiah will come with such gentleness and humility that He will not break even a bruised reed on the ground. In this surprising way, He will bring justice to victory.

Isaiah's description fits Jesus perfectly. Instead of fighting off the Romans, Jesus will submit to their execution of Him. He will die to pay for the sins of others, allowing all who trust in Him to be forgiven for their sins and declared righteous by God. They will experience victorious justice through submitting to the Savior who submitted to death and was resurrected in victory.

Finally, in this verse, Matthew points to Isaiah to show that the Gentiles will hope in the name of the Messiah. Instead of defeating the Gentiles and sending them away, Jesus will become the way in which the people of the non-Jewish nations come in hope to God the Father.