John Chrysostom

It follows, I came not to call the righteous, but sinners to repentance. As if He should say, So far am I from hating sinners, that for their sakes only I came, not that they should remain sinners, but be converted and become righteous. Now He speaks of the righteous ironically, as when He says, Behold Adam is become as one of us. (Gen. 3:22.) But that there was none righteous upon the earth St. Paul shows, saying, All have sinned, and need the grace of God. (Rom. 3:23.)

Catena Aurea by Aquinas ↗

Augustine of Hippo

Hence He adds, to repentance, which serves well to explain the passage, that no one should suppose that sinners, because they are sinners, are loved by Christ, since that similitude of the sick plainly suggests what our Lord meant by calling sinners, as a Physician, the sick, in order that from iniquity as from sickness they should be saved.

Catena Aurea by Aquinas ↗

Ambrose of Milan

But how does God love righteousness, and David has never seen the righteous man forsaken, if the righteous are excluded, the sinner called; unless you understand that He meant by the righteous those who boast of the law, (Ps. 11:7, Ps. 37:25.) and seek not the grace of the Gospel. Now no one is justified by the law, but redeemed by grace. He therefore calls not those who call themselves righteous, for the claimers to righteousness are not called to grace. …

Catena Aurea by Aquinas ↗

Bede

"I have not come to call the just but sinners" can also be properly understood in this way. He has not called those who, wishing to establish their own justice, have not been made subject to the justice of God. He calls those who, being conscious of their weakness, are not ashamed to confess that we have all offended in many things. In them too is fulfilled his saying that he had not come to call the just but sinners. That is, he does not call the exalted but the humble. …

Homilies on the Gospels 1.21 ↗