Matthew 5:9

Blessed are the peacemakers--who not only study peace, but diffuse it. for they shall be called the children of God--shall be called sons of God. Of all these beatitudes this is the only one which could hardly be expected to find its definite ground in the Old Testament; for that most glorious character of God, the likeness of which appears in the peacemakers, had yet to be revealed. His glorious name, indeed--as "The Lord, the Lord God, merciful and gracious, long-suffering, and abundant in goodness and truth, forgiving iniquity and transgression and sin"--had been proclaimed in a very imposing manner (Exo 34:6), and manifested in action with affecting frequency and variety in the long course of the ancient economy. And we have undeniable evidence that the saints of that economy felt its transforming and ennobling influence on their own character. But it was not till Christ "made peace by the blood of the cross" that God could manifest Himself as "the God of peace, that brought again from the dead our Lord Jesus, that great Shepherd of the sheep, through the blood of the everlasting covenant" (Heb 13:20) --could reveal Himself as "in Christ reconciling the world unto Himself, not imputing their trespasses unto them," and hold Himself forth in the astonishing attitude of beseeching men to be "reconciled to Himself" (Co2 5:19-20).

— Jamieson-Fausset-Brown Commentary (public domain)