St. Thomas Aquinas on Ministering to Sinners

9. And when Jesus passed on from thence, he saw a man sitting in the custom house, named Matthew; and he saith to him: Follow me. And he arose up and followed him. 10. And it came to pass as he was sitting at meat in the house, behold many publicans and sinners came, and sat down with Jesus and his disciples. 11. And the Pharisees seeing it, said to his disciples: Why doth your master eat with publicans and sinners? 12. But Jesus hearing it, said: They that are in health need not a physician, but they that are ill. 13. Go then and learn what this meaneth, I will have mercy and not sacrifice. For I am not come to call the just, but sinners. (Matthew 9:9-13)

[...] Therefore, when the Pharisees saw this, they said to his disciples. It should be noted that those Pharisees were malicious; hence they wished to effect a rupture between the disciples and Jesus. So they complained about Jesus to the disciples, and about the disciples to Jesus. Therefore, wishing to complain to the disciples about Jesus, they ask: Why does your teacher eat with tax collectors and sinners? They belong to that group mentioned in Proverbs (6:16): "There are six things which the Lord hates, and a seventh which his soul detests," namely, "the man who sows discord among brothers." But why does Luke say that this was said about the disciples? Augustine answers that the sentiment of both is the same, although the words are different, because they imputed it all to the teaching of the Master. Hence Luke refers to the words, Matthew to the sentiment. But it seems that they were right, because association with sinners should be avoided.

1. However, it should be noted that association with sinners must be avoided sometimes on account of pride and contempt, as those in Isaiah (65:5): "Keep to yourself, do not come near me, because you are unclean."
2. But others avoid the company of sinners for the welfare of the sinners, so that they will be ashamed and be converted; thus it is that Paul says in 1 Corinthians (6:5): "I say this to your shame. Is there no man among you wise enough to decide?"
3. Likewise, others avoid them as a precaution, fearing that they be defiled: "Whoever touches pitch will be defiled" (Sir 13:11); "With the crooked you show yourself perverse" (Ps 18:26).

On the other hand, some mingle with sinners to prove themselves; hence temptation is a test of oneself, as it says in Sirach (27:6); and 2 Peter (2:8) says: "By what the just man saw and heard as he lived among them, he was vexed in his righteous soul"; and Song of Songs (2:2): "As a lily among thorns, so is my love among maidens." And a Gloss: "He was not good who could not endure evil men." Some also mingle with evil men to convert them: "I have become all things to all men, that I might gain all" (1 Cor 9:9). But there is a difference, because it is not fitting to communicate with persistent sinners and those who refuse to repent. But in regard to those about whom there is hope, we must distinguish on the part of the one mingling with sinners whether he is strong or weak. If he is weak, he should not mingle with them; if he is strong, it is suitable for him to remain in order to convert them to God. Furthermore, Jesus the Physician was certain; consequently, while he was with them, there was no danger to fear...

(St. Thomas Aquinas, Commentary on Saint Matthew's Gospel (Super Evangelium S. Matthaei lectura), trans. R.F. Larcher, chapter 9, lecture 2, verse 11; http://dhspriory.org/thomas/SSMatthew.htm.)

Emphases mine.