From: Matthew 5:43-48
Jesus and His Teaching, the Fulfillment of the Law (Continuation)
(Jesus said to His disciples,)  "You have heard that it was said, `You shall love your neighbor and hate your enemy.'  But I say to you, Love your enemies and pray for those who persecute you.  So that you may be sons of your Father who is in Heaven; for He makes His sun rise on the evil and on the good, and sends rain on the just and on the unjust.  For if you love those who love you, what reward have you? Do not even the tax collectors do the same?  And if you salute only your brethren, what more are you doing than others? Do not even the Gentiles do the same?  You, therefore, must be perfect, as your Heavenly Father is perfect."
43. The first part of this verse--"You shall love your neighbor"--is to be found in Leviticus 19:18. The second part--"hate your enemy"--is not to be found in the Law of Moses. However, Jesus' words refer to a widespread rabbinical interpretation which understood "neighbors" as meaning "Israelites". Our Lord corrects this misinterpretation of the Law: for Him everyone is our neighbor (cf. the parable of the Good Samaritan in Luke 10:25-37).
43-47. This passage sums up the teaching which precedes it. Our Lord goes so far as to say that a Christian has no personal enemies. His only enemy is evil as such--sin--but not the sinner. Jesus Himself puts this into practice with those who crucified Him, and He continues to act in the same way towards sinners who rebel against Him and despise Him. Consequently, the saints have always followed His example--like St. Stephen, the first martyr, who prayed for those who were putting him to death. This is the apex of Christian perfection--to love, and pray for, even those who persecute us and calumniate us. It is the distinguishing mark of the children of God.
46. "Tax collectors": the Roman empire had no officials of its own for the collection of taxes: in each country it used local people for this purpose. These were free to engage agents (hence we find reference to "chief tax collectors": cf. Luke 19:2). The global amount of tax for each region was specified by the Roman authorities; the tax collectors levied more than this amount, keeping the surplus for themselves: this led them to act rather arbitrarily, which was why the people hated them. In the case of the Jews, insult was added to injury by the fact that the chosen people were being exploited by Gentiles.
48. Verse 48 is, in a sense, a summary of the teaching in this entire chapter, including the Beatitudes. Strictly speaking, it is quite impossible for a created being to be as perfect as God. What our Lord means here is that God's own perfection should be the model which every faithful Christian tries to follow, even though he realizes that there is an infinite distance between himself and his Creator. However, this does not reduce the force of this commandment; it sheds more light on it. It is a difficult commandment to live up to, but along with this we must take account of the enormous help grace gives us to go so far as to tend towards divine perfection. Certainly, perfection which we should imitate does not refer to the power and wisdom of God, which are totally beyond our scope; here the context seems to refer primarily to love and mercy. Along the same lines, St. Luke quotes these words of our Lord: "Be merciful, even as your Father is merciful" (Luke 6:36; cf. note on Luke 6:20-49).
Clearly, the "universal call to holiness" is not a recommendation but a commandment of Jesus Christ.
"Your duty is to sanctify yourself. Yes, even you. Who thinks that this task is only for priests and religious? To everyone, without exception, our Lord said: `Be ye perfect, as My Heavenly Father is perfect'" ([St] J. Escriva, "The Way", 291). This teaching is sanctioned by chapter 5 of Vatican II's Constitution "Lumen Gentium", where it says (40): "The Lord Jesus, divine teacher and model of all perfection, preached holiness of life (of which He is the author and maker) to each and every one of His disciples without distinction: `You, therefore, must be perfect, as your Heavenly Father is perfect' [...]. It is therefore quite clear that all Christians in any state or walk of life are called to the fullness of Christian life and to the perfection of love, and by this holiness a more human manner of life is fostered also in earthly society."
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.