8 Hear and understand, if haply through His help I shall be able to explain it; and may He help us to the understanding of all the above-named works and rewards, how suitable rewards are apportioned to their corresponding duties. For where is there anything said of a reward which does not suit, and harmonize with its work? Because the lowly seem as it were aliens from a kingdom, He says, “Blessed are the poor in spirit, for theirs is the kingdom of heaven.” Because meek men are easily despoiled of their land, He says, “Blessed are the meek, for they shall inherit the land.” Now the rest are plain at once; they are understood of themselves, and require no one to treat of them at length; they need only one to mention them.
“Blessed are they that mourn.” Now what mourner does not desire consolation? “They,” says He, “shall be comforted.” “Blessed are they that hunger and thirst after righteousness.” What hungry and thirsty man does not seek to be filled? “And they,” says He, “shall be filled.” “Blessed are the merciful.” What merciful man but wishes that a return should be rendered him by God of His own work, that it may be so done to him, as he does to the poor? “Blessed,” says He, “are the merciful, for they shall obtain mercy.”
How in each case has every duty its appropriate reward: and nothing is introduced in the reward which does not suit the precept! For the precept is, that you be “poor in spirit;” the reward, that you shall have the “kingdom of heaven.” The precept is, that you be “meek;” the reward, that you shall “possess the earth.” The percept is, that you “mourn;” the reward, that you shall be “comforted.” The precept is, that you “hunger and thirst after righteousness;” the reward, that you shall “be filled.” The precept is, that you be “merciful;” the reward, that you shall “obtain mercy.” And so the precept is, that you cleanse the heart; the reward, that you shall see God.
Source: Sermons on Selected Lessons of the New Testament (New Advent)