From: Matthew 9:9-13
The Call of Matthew
 As Jesus passed on from there, He saw a man called Matthew sitting at the tax office; and He said to him, "Follow Me." And he rose and followed Him.
 And as He sat at table in the house, behold, many tax collectors and sinners came and sat down with Jesus and His disciples.  And when the Pharisees saw this, they said to His disciples, "Why does your Teacher eat with tax collectors and sinners?"  But when He heard it, He said, "Those who are well have no need of a physician, but those who are sick.  Go and learn what this means, `I desire mercy, and not sacrifice.' For I came not to call the righteous, but sinners."
9. "Tax office": a public place for the payment of taxes. On "following Jesus", see the note on Matthew 8:18-22.
The Matthew whom Jesus calls here is the Apostle of the same name and the human author of the first Gospel. In Mark 2:14 and Luke 5:27 he is called Levi the son of Alphaeus or simply Levi.
In addition to Baptism, through which God calls all Christians (cf. note on Matthew 8:18-22), the Lord can also extend, to whomever He chooses, a further calling to engage in some specific mission in the Church. This second calling is a special grace (cf. Matthew 4:19-21; Mark 1:17-20; John 1:30; etc.) additional to the earlier calling through Baptism. In other words, it is not man who takes the initiative; it is Jesus who calls, and man who responds to this call by his free personal decision: "You did not choose Me, but I chose you" (John 15:16).
Matthew's promptitude in "following" Jesus' call is to be noted. When God speaks, soul may be tempted to reply, "Tomorrow; I'm not ready yet." In the last analysis this excuse, and other excuses, are nothing but a sign of selfishness and fear (different from that fear which can be an additional symptom of vocation: cf. John 1). "Tomorrow" runs the risk of being too late.
As in the case of the other Apostles, St. Matthew is called in the midst of the ordinary circumstances of his life: "What amazes you seems natural to me: that God has sought you out in the practice of your profession! That is how He sought the first, Peter and Andrew, James and John, beside their nets, and Matthew, sitting in the custom-house. And--wonder of wonders!--Paul, in his eagerness to destroy the seed of the Christians" ([St] J. Escriva, "The Way", 799).
10-11. The attitude of these Pharisees, who are so prone to judge others and classify them as just men or sinners, is at odds with the attitude and teaching of Jesus. Earlier on, He said, "Judge not, that you be not judged" (Matthew 7:1), and elsewhere He added, "Let him who is without sin among you be the first to throw a stone at her" (John 8:7).
The fact is that all of us are sinners; and our Lord has come to redeem all of us. There is no basis, therefore, for Christians to be scandalized by the sins of others, since any one of us is capable of committing the vilest of sins unless God's grace comes to our aid.
12. There is no reason why anyone should be depressed when he realizes he is full of failings: recognition that we are sinners is the only correct attitude for us to have in the presence of God. He has come to seek all men, but if a person considers himself to be righteous, by doing so he is closing the door to God; all of us in fact are sinners.
13. Here Jesus quotes Hosea 6:6, keeping the hyperbole of the Semitic style. A more faithful translation would be: "I desire mercy MORE THAN sacrifice". It is not that our Lord does not want the sacrifices we offer Him: He is stressing that every sacrifice should come from the heart, for charity should imbue everything a Christian does--especially his worship of God (see 1 Corinthians 13:1-13; Matthew 5:23-24).
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.