From: Mark 2:13-17
The Calling of Matthew
 He (Jesus) went out again beside the sea; and all the crowd gathered about Him, and He taught them.  And as He passed on, He saw Levi the son of Alphaeus 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 his house, many tax collectors and sinners were sitting with Jesus and His disciples; for there were many who followed Him.  And the scribes of the Pharisees, when they saw that He was eating with sinners and tax collectors, said to His disciples, "Why does He eat with tax collectors and sinners?"  And when Jesus heard it, He said to them, "Those who are well have no need of a physician, but those who are sick; I came not to call the righteous, but sinners."
14. St. Mark and St. Luke (5:27-32) both call him "Levi"; the First Gospel, on the other hand, calls him "Matthew" (Matthew 9:9-13); but they are all referring to the same person. All three accounts describe the same event. Later on, St. Mark and St. Luke, when giving the list of Apostles (Mark 3:13-19; Luke 6:12-16), include Matthew, not Levi. The Fathers identify Matthew with Levi. Besides it was quite common for Jews to have two names: Jacob-Israel, Simon-Peter, Saul-Paul, Joseph-Caiaphas, John-Mark... Frequently, the name and surname were connected with some significant change in the life and mission of the person concerned. Did Jesus' saving intervention in this Apostle's life lead to a change of name? The Gospel does not tell us.
Levi-Matthew, as a publican or tax collector (Matthew 9:9-13), was sitting at the `tax office', a special place where one went to pay tribute. Publicans were tax-collectors appointed by the Romans. It was, therefore, an occupation hated and despised by the people; but it was also a much-coveted position because it was an easy way to become prosperous. Matthew leaves everything behind when Jesus calls him. He immediately responds to his vocation, because Jesus gives him the grace to accept his calling.
Jesus is the basis of our confidence in being able to change, provided we cooperate with His grace, no matter how unworthy our previous conduct may have been. And He is also the source of the confidence we need in order to be apostolic--helping others to be converted and seek holiness of life. Because He is the Son of God He is able to raise up children of God even from stones (cf. Matthew 3:9). Cf. note on Matthew 9:9.
17. The scribes and Pharisees reproach the disciples, and Jesus replies with a popular proverb: `Those who are well have no need of a physician, but those who are sick.' He is the doctor of souls, come to cure sinners of their spiritual ailments.
Our Lord calls everyone, His redemptive mission extends to everyone; He affirms this on other occasions, using parables such as that of the marriage feast (Matthew 22:1-14; Luke 14:16-24). How, then, can we explain the restriction He seems to place here by saying that He has not come to call the righteous? It is not really a restriction. Jesus uses the opportunity to reproach the scribes and Pharisees for their pride: they consider themselves just, and their reliance on their apparent virtue prevents them from hearing the call to conversion; they think they can be saved by their own efforts (cf. John 9:41). This explains the proverb Jesus quotes; certainly His preaching makes it quite clear that `no one is good but God alone' (Mark 10:18) and that everyone must have recourse to the mercy and forgiveness of God in order to be saved. In other words, mankind is not divided into two--the just and the unjust. We are all sinners, as St. Paul confirms: `all have sinned and fall short of the glory of God' (Romans 3:23). Precisely because of this, Christ came to call all of us; He justifies those who respond to His call.
Our Lord's words should also move us to pray humbly and confidently for people who seem to want to continue living in sin. As St. Teresa beseeched God: "Ah, how hard a thing am I asking of Thee, my true God! I ask Thee to love one who loves Thee not, to open to one who has not called upon Thee, to give health to one who prefers to be sick and who even goes about in search of sickness. Thou sayest, my Lord, that Thou comest to seek sinners; these, Lord, are the true sinners. Look not upon our blindness, my God, but upon all the blood that was shed for us by Thy Son. Let Thy mercy shine out amid such tremendous wickedness. Behold, Lord, we are the works of Thy hands" ("Exclamations of the Soul to God", n. 8).
The Fathers of the Church see this calling by Jesus as an invitation to repentance and penance. St. John Chrysostom ("Hom. on St. Matthew", 30:3), for example, explains the phrase by putting these words in Jesus' mouth: "I am not come that they should continue sinners but that they should change and become better."
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.