“Blessed are the merciful: for they shall obtain mercy” (Matt 5:7).
“Justice and mercy are so united, that the one ought to be mingled with the other; justice without mercy is cruelty; mercy without justice, profusion.” -Glossa Ordinaria
“The merciful is he who has a sad heart; he counts others’ misery his own, and is sad at their grief as at his own.” -Remigius of Auxerre
“Mercy here is not said only of alms, but is in every sin of a brother, if we bear one another’s burdens.” -St. Jerome
“He pronounces those blessed who succour the wretched, because they are rewarded in being themselves delivered from all misery; as it follows, ‘for they shall obtain mercy.'” -St. Augustine
“So greatly is God pleased with our feelings of benevolence towards all men, that He will bestow His own mercy only on the merciful.” -St. Hilary of Poitiers
“The reward here seems at first to be only an equal return; but indeed it is much more; for human mercy and divine mercy are not to be put on an equality.” -St. John Chrysostom
“Justly is mercy dealt out to the merciful, that they should receive more than they had deserved; and as he who has more than enough receives more than he who has only enough, so the glory of mercy is greater than of the things hitherto mentioned.” -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).