From a sermon on the beatitudes by Saint Leo the Great, pope
The blessedness of seeing God is justly promised to the pure of heart. For the eye that is unclean would not be able to see the brightness of the true light, and what would be happiness to clear minds would be a torment to those that are defiled. Therefore, let the mists of worldly vanities be dispelled, and the inner eye be cleansed of all the filth of wickedness, so that the soul’s gaze may feast serenely upon the great vision of God.
It is to the attainment of this goal that the next words refer: Blessed are the peacemakers, for they shall be called sons of God. This blessedness, dearly beloved, does not derive from any casual agreement or from any and every kind of harmony, but it pertains to what the Apostle says: Be at peace before the Lord, and to the words of the prophet: Those who love your law shall enjoy abundant peace; for them it is no stumbling block.
Even the most intimate bonds of friendship and the closest affinity of minds cannot truly lay claim to this peace if they are not in agreement with the will of God. Alliances based on evil desires, covenants of crime and pacts of vice–all lie outside the scope of this peace. Love of the world cannot be reconciled with love of God, and the man who does not separate himself from the children of this generation cannot join the company of the sons of God. But those who keep God ever in their hearts, and are anxious to preserve the unity of the spirit in the bond of peace, never dissent from the eternal law as they speak the prayer of faith. Your will be done on earth as it is in heaven.
These then are the peacemakers; they are bound together in holy harmony and are rightly given the heavenly title of sons of God, co- heirs with Christ. And this is the reward they will receive for their love of God and neighbor: when their struggle with all temptation is finally over, there will be no further adversities to suffer or scandal to fear; but they will rest in the peace of God undisturbed, through our Lord who lives and reigns with the Father and the Holy Spirit for ever and ever. Amen.