This is one of Jesus’ most important AND most misunderstood teachings. The hypocritical judgments of the Pharisees and teachers of the law were a constant target of Jesus’ criticisms. To judge another not on the basis of their heart but on superficial assumptions was absolutely sinful in Jesus’ eyes.
At the same time, Jesus constantly warned his followers about false teachers. Without some means of judgment, how were they to know who these teachers were? As with all things in the Sermon on the Mount, the key is our relationship to our Heavenly Father. God’s Spirit of Truth will guide us in making discerning and fair judgments about others. This same Spirit will lead us to judge not with a vitriolic attitude of condemnation but with a merciful and loving desire for the good of all.