Matthew 5:9 is part of Jesus’ Sermon on the Mount in which He says, "Blessed are the peacemakers, for they shall be called the children of God." The Greek word translated "peacemaker" is used in only one other place in the New Testament, in a slightly different form. Colossians 1:20 says, "For it was the Father’s good pleasure for all the fullness to dwell in Him, and through Him to reconcile all things to Himself, having made peace through the blood of His cross."
Jesus laid down His life to make peace between God and sinners, and when we can carry that message of peace to others, we are peacemakers. God delights in those who reconcile others to Himself—those who bring the gospel are “beautiful” (Isaiah 52:7). God “reconciled us to himself through Christ and gave us the ministry of reconciliation” (2 Corinthians 5:18). Those who bring reconciliation to broken relationships are carrying on the work of Jesus, the Prince of Peace. Those who give of themselves as Jesus did in order that others may know God are called "blessed." There is no real peace apart from a relationship with God (Romans 5:1). What may masquerade as worldly peace is merely a temporary lull in chaos (John 14:27). True peace is found only in a restored relationship with God. “‘There is no peace,’ says the LORD, ‘for the wicked’” (Isaiah 48:22).
Only children of God can bring the peace of knowing God to others. A person must have a real relationship with God before he or she can help someone else know God. Those who witness for Christ, share their faith with their friends, and serve others in the name of Christ are the ambassadors for peace this verse identifies (see also Matthew 10:41-42). Those who bring the wonderful message of God’s peace to the world are "peacemakers," and Jesus calls them the "children of God."