You can judge another based on conduct. They either do or do not do something. The conduct is observable, and therefore capable of being judged. Now, however, Christ moves the battleground inside a person. It is now in the heart. On such terrain as that, man is incapable of knowing, and therefore, of judging.
If you can prevent this at the heart, you can heal society. Refrain from letting offenses turn into anger. Deal with them inside, showing forgiveness and compassion. He will stress this further in subsequent verses.