The Lord now corrects a fault that easily disguises itself as righteousness: the rash judging of others. He does not forbid all discernment, but condemns that judgment which proceeds from pride rather than charity. For when a man judges his brother harshly, he reveals not clarity of sight, but blindness to his own condition.

The measure by which one judges is the interior standard one applies to oneself. Those who judge without mercy prepare judgment for themselves, because they have chosen severity rather than compassion. God repays according to the rule a man has embraced, not arbitrarily, but justly.

The image of the mote and the beam exposes the hypocrisy of self-deception. Small faults in others appear large when one’s own greater faults are ignored. The beam signifies pride itself, which prevents the soul from seeing clearly. Until this is removed, even attempts to correct others are harmful, because they are undertaken without humility.

Correction is not forbidden; it is purified. Once the heart is cleansed of pride, one may see clearly enough to help another. But this clarity is born of repentance, not superiority. True correction flows from shared weakness, not assumed innocence.

The Lord then adds a caution that balances mercy with prudence. What is holy must not be offered indiscriminately to those who will only despise it. This is not contempt for persons, but discernment regarding readiness. For sacred teaching, when forced upon the unwilling or the hostile, is not only wasted but may provoke greater sin.

Thus the soul is taught to walk between two dangers: harsh judgment on the one hand, and imprudent generosity on the other. Charity judges itself before judging others, and wisdom knows when to speak and when to remain silent. In this way, righteousness is preserved without cruelty, and mercy is exercised without folly.

Source: St. Augustine, On the Lord’s Sermon on the Mount,  Book II, Chapters 15-17