St Augustine begins his book on Our Lord’s Sermon on the Mount by the example in Matthew of the “perfect standard of the Christian life:”
Everyone then who hears these words of mine and does them
will be like a wise man who built his house upon the rock;
and the rain fell, and the floods came,
and the winds blew and beat upon that house,
but it did not fall, because it had been founded on the rock.
And every one who hears these words of mine and does not do them
will be like a foolish man who built his house upon the sand;
and the rain fell, and the floods came,
and the winds blew and beat against that house,
and it fell; and great was the fall of it.
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Who are the poor in spirit in the first beatitude, “Blessed are you poor, for yours is the kingdom of God?” St Augustine teaches us that the poor in spirit are those who “are humble and God fearing,” whose spirit is not proud, not puffed up by the wind. St Augustine contrasts this with the spirit of the proud, in a passage quoted by the Catholic Catechism, that though the “proud seek after and love the kingdoms of the earth” while the poor in spirit possess the kingdom of heaven.
In the beatitude, “Blessed are those who mourn, for they shall be comforted,” those who mourn, losing their temporal joy, are comforted by the Holy Spirit may gain eternal joy.
Who are the meek in the beatitude, “Blessed are the meek, for they shall inherit the earth?” St Augustine teaches us that the “meek are those who do not yield to acts of wickedness, and do not resist evil, but overcome evil with good. Let those who are not meek quarrel and fight for earthly and temporal things,” while the meek “by inheritance shall possess the earth from which they cannot be driven out.” As in Romans, “Do not be overcome by evil, but overcome evil with good.”
Next is the beatitude, “Blessed are those who hunger and thirst for righteousness, for they shall be satisfied.” Here St Augustine references the story of Jesus says and the Samaritan woman at the well. Jesus asks her for a drink, and she asks Jesus why He, as a Jew, would ask a Samaritan for a drink, when Jews have no dealings with Samaritans. Jesus responds that if she only knew who He was, she would ask Him for living water. “Whoever drinks of the water that I shall give him will never thirst; the water that I shall give him will become in him a spring of water welling up to eternal life.” Later in this chapter the disciples bid Jesus to eat, and Jesus responds, “My food is to do the will of him who sent me, and to accomplish his work.”
Next is the beatitude, “Blessed are the merciful, for they shall obtain mercy.” How we expect others to show us mercy for our faults, yet we shirk from showing mercy in judging the faults of others.
Ascending to the sixth beatitude, which exhorts, “Blessed are the pure in heart, for they shall see God.” St Augustine teaches that the pure in heart are “able from a good conscience of good works to contemplate the highest good, which can be discerned by the pure and tranquil intellect alone.” Here St Augustine may be borrowing the platonic conception of the contemplation of the good. Do not seek God with outward eyes, seek to see God with the eyes of the heart!
“Blessed are the peacemakers, for they shall be called sons of God.” St Augustine teaches that we must first be peacemakers in the battle in our own soul, subjecting the motions of our soul to reason, subduing our carnal lusts which we share with the wild beasts, casting out the prince of this world, curing our perversity and disorder.
We come full circle with the beatitude, “Blessed are those who are persecuted for righteousness’ sake, for theirs is the kingdom of heaven,” for both the poor in spirit and the persecuted possess the kingdom of heaven. St Augustine recalls the verse in Romans, “We rejoice in our sufferings, knowing that suffering produces endurance, and endurance produces character, and character produces hope, and hope does not disappoint us, because God’s love has been poured into our hearts through the Holy Spirit who has been given to us.” St Augustine teaches us that not only should we endure persecution, but we should endure it with a tranquil spirit, but “even with exultation.”
“Blessed are you when men revile you and persecute you and utter all kinds of evil against you falsely on my account. Rejoice and be glad, for your reward is great in heaven, for so men persecuted the prophets who were before you.” St Augustine teaches that in violent times our lives are threatened when we are persecuted, but that in more peaceful times we may be persecuted by words and lies and by attempts to ruin our reputation, but our reward is great in heaven, “where dwells everlasting righteousness,” where the mortal becomes immortal, the heaven where we “will be perfected in every part.”
 Catholic Catechism, Section 2547, http://ccc.usccb.org/flipbooks/catechism/index.html#608
 St Augustine, “Our Lord’s Sermon On The Mount,” In the Nicene and Post-Nicene Fathers, First Series, Volume 6, translated by Rev CL Cornish (Boston: Hendrickson Publishers, 1994, first published 1887), pp. 3-8.