From: Luke 6:27-38
Love of Enemies
 "But I say to you that hear, Love your enemies, do good to those who hate you,  bless those who curse you, pray for those who abuse you.  To him who strikes you on the cheek, offer the other also; and from him who takes away your cloak do not withhold your coat as well.  Give to every one who begs from you; and of him who takes away your goods do not ask them again.  And as you wish that men would do to you, do so to them.
 "If you love those who love you, what credit is that to you? For even sinners love those who love them.  And if you do good to those who do good to you, what credit is that to you? For even sinners do the same.  And if you lend to those from whom you hope to receive, what credit is that to you? Even sinners lend to sinners, to receive as much again.  But love your enemies, and do good, and lend, expecting nothing in return; and your reward will be great, and you will be sons of the Most High, for he is kind to the ungrateful and the selfish.  Be merciful, even as your Father is merciful.
 "Judge not, and you will not be judged; condemn not, and you will not be condemned; forgive, and you will be forgiven;  give, and it will be given to you; good measure, pressed down, shaken together, running over, will be put into your lap. For the measure you give will be the measure you get back."
27. "In loving our enemies there shines forth in us some likeness to God our Father, who, by the death of His Son, ransomed from everlasting perdition and reconciled to Himself the human race, which previously was most unfriendly and hostile to Him" ("St. Pius V Catechism", IV, 14, 19). Following the example of God our Father, we must desire for everyone (even those who say they are our enemies) eternal life, in the first place; additionally, a Christian has a duty to respect and understand everyone without exception, because of his or her intrinsic dignity as a human person, made in the image and likeness of the Creator.
28. Jesus Christ teaches us by example that this is a real precept and not just a pious recommendation; even when nailed to the cross He prayed to His Father for those who had brought Him to such a pass: "Father, forgive them, for they know not what they do" (Lk 23:34). In imitation of the Master, St Stephen, the first martyr of the Church, when he was being stoned, prayed to our Lord not to hold the sin against his persecutors (cf. Acts 7:60). In the liturgy of Good Friday the Church offers prayers and suffrages to God on behalf of those outside the Church, asking Him to give them the grace of faith; to release from their ignorance those who do not know Him; to give Jews the light to the truth; to bring non-Catholic Christians, linked by true charity, into full communion with our Mother the Church.
29. Our Lord gives us more examples to show us how we should act if we want to imitate the mercy of God. The first has to do with one of what are traditionally called the "spiritual works of mercy"--forgiving injuries and being patient with other people's defects. This is what He means in the first instance about turning the other cheek.
To understand what our Lord is saying here, St. Thomas comments that "Sacred Scripture needs to be understood in the light of the example of Christ and the saints. Christ did not offer the cheek to be struck in the house of Annas (Jn 18:22ff), nor did St. Paul when, as we are told in the Acts of the Apostles, he was beaten in Philippi (Acts 16:22f). Therefore, we should not take it that Christ literally meant that you should offer the other cheek to some to hit you; what He was referring to was your interior disposition; that is, if necessary we should be ready not to be intolerant of anyone who hurts us, and we should be ready to put up with this kind of treatment, or worse than that. That was how the Lord acted when He surrendered His body to death" ("Commentary on St John", 18, 37).
36. The model of mercy which Christ sets before us is God Himself, of whom St. Paul says, 'Blessed be the God and Father of our Lord Jesus Christ, the Father of mercies and God of all comfort, who comforts us in all our afflictions" (2 Cor 1:3-4). "The first quality of this virtue", Fray Luis de Granada explains, "is that it makes men like God and like the most glorious thing in Him, His mercy (Lk 6:36). For certainly the greatest perfection a creature can have is to be like his Creator, and the more like Him he is, the more perfect he is. Certainly one of the things which is most appropriate to God is mercy, which is what the Church means when it says that prayer: 'Lord God, to whom it is proper to be merciful and forgiving...'. It says that this is proper to God, because just as a creature, as creature, is characteristically poor and needy (and therefore characteristically receives and does not give), so, on the contrary, since God is infinitely rich and powerful, to Him alone does it belong to give and not to receive, and therefore it is appropriate for Him to be merciful and forgiving" ("Book of Prayer and Meditation", third part, third treatise).
This is the rule a Christian should apply: be compassionate towards other people's afflictions as if they were one's own, and try to remedy them. The Church spells out this rule by giving us a series of corporal works of mercy (visiting and caring for the sick, giving food to the hungry, drink to the thirsty...) and spiritual works of mercy (teaching the ignorant, correcting the person who has erred, forgiving injuries...): cf. "St Pius X Catechism", 944f.
We should also show understanding towards people who are in error: "Love and courtesy of this kind should not, of course, make us indifferent to truth and goodness. Love, in fact, impels the followers of Christ to proclaim to all men the truth which saves. But we must distinguish between the error (which must always be rejected) and the person in error, who never loses his dignity as a person even though he flounders amid false or inadequate religious ideas. God alone is the judge and searcher of hearts; He forbids us to pass judgment on the inner guilt of others" (Vatican II, "Gaudium Et Spes", 28).
38. We read in Sacred Scripture of the generosity of the widow of Zarephath, whom God asked to give food to Elijah the prophet even though she had very little left; He then rewarded her generosity by constantly renewing her supply of meal and oil (1 Kings 17:9ff). The same sort of thing happened when the boy supplied the five loaves and two fish which our Lord multiplied to feed a huge crowd of people (cf. Jn 6:9)--a vivid example of what God does when we give Him whatever we have, even if it does not amount to much.
God does not let Himself be outdone in generosity: "Go, generously and like a child ask Him, 'What can You mean to give me when You ask me for "this"?'" ([St] J. Escriva, "The Way", 153). However much we give God in this life, He will give us more in life eternal.
Source: "The Navarre Bible: Text and Commentaries". Biblical text taken from the Revised Standard Version and New Vulgate. Commentaries made by members of the Faculty of Theology of the University of Navarre, Spain. Published by Four Courts Press, Kill Lane, Blackrock, Co. Dublin, Ireland. Reprinted with permission from Four Courts Press and Scepter Publishers, the U.S. publisher.