“Blessed are the peacemakers: for they shall be called the children of God” (Matt 5:9).
“When you have made your inward parts clean from every of sin, that dissentions and contentions may not proceed from your temper, begin peace within yourself, that so you may extend it to others.” -St. Ambrose
“Peace is the fixedness of order; by order, I mean an arrangement of things like and unlike giving to each its own place. And as there is no man who would not willingly have joy, so is there no man who would not have peace; since even those who go to war desire nothing more than by war to come to a glorious peace.” -St. Augustine
“The peacemakers are pronounced blessed, they namely who make peace first within their own hearts, then between brethren at variance. For what avails it to make peace between others, while in your own heart are wars of rebellious vices.” -St. Jerome
“The peacemakers within themselves are they who having stilled all disturbances of their spirits, having subjected them to reason, have overcome their carnal desires, and become the kingdom of God. There all things are so disposed, that that which is most chief and excellent in man, governs those parts which we have in common with the brutes, though they struggle against it; nay even that in man which is excellent is subjected to a yet greater, namely, the very Truth, the Son of God. For it would not be able to govern what is inferior to it, if it were not subject to what is above it. And this is the peace which is given on earth to men of good will.” -St. Augustine
“No man can attain in this life that there be not in his members a law resisting the law of his mind. But the peacemakers attain thus far by overcoming the lusts of the flesh, that in time they come to a most perfect peace.” -St. Augustine
“The peacemakers with others are not only those who reconcile enemies, but those who unmindful of wrongs cultivate peace. That peace only is blessed which is lodged in the heart, and does not consist only in words. And they who love peace, they are the sons of peace.” -Pseudo-Chrysostom
“The blessedness of the peacemakers is the reward of adoption, ‘they shall be called the sons of God.’ For God is our common parent, and no other way can we pass into His family than by living in brotherly love together.” -St. Hilary of Poitiers
“Because peace is then perfect when there is nowhere any opposition, the peacemakers are called the sons of God, because nothing resists God, and the children ought to bear the likeness of their Father.” -St. Augustine
“The peacemakers have thus the place of highest honour, inasmuch as he who is called the king’s son, is the highest in the king’s house. This beatitude is placed the seventh in order, because in the Sabbath shall be given the repose of true peace, the six ages being passed away.” -Glossa apud Anselm.
Quotations from St. Thomas Aquinas, Catena Aurea: Commentary on the Four Gospels Collected Out of the Works of the Fathers, Vol. I, Part I (Oxford: John Henry Parker, 1841).