These from his book “The Sermon on the Mount“, available through CPH.
On verses 15-20:
The false prophets came disguised as legitimate bearers of the message of Jesus. Such a description does not seem to fit Jesus’ contemporaries, who clearly identified themselves as his adversaries. The warning against the false prophets is basically an anticipatory and eschatological word of Jesus projected into the community of his followers. The period of the false prophets comes when Jesus is no longer with them. It is the time between his departure from them and his appearance as the eschatological judge.
Of all the words used for the pastoral or clerical office in the New Testament, “prophet” was not used much beyond apostolic times. In Didache 11:3, it was still used of itinerant preachers. It can be taken as a general warning for any office which is entrusted with expounding the divine Word. Thus the false prophet wants to be understood as an authoritative spokesman for the message of Jesus within his community, but he has a message diametrically opposed to Jesus’.
Jesus uses two analogies in his discussion of the false prophets. The first involves animals and the second plants. In the first case the wolves appear in sheep’s clothing; a better translation might be sheepskins. Old Testament prophets dressed in sheepskins. Before the kill, the wolf gets in as close to the flock as it possibly can without being detected. The false teacher does the same thing in the community of the followers of Jesus. He remains undetected among the flock and thus causes no panic.
In the phrase, “You will know them by their fruits,” fruits refer to what not to expect of false teachers. The gathering of grapes from thorns and figs from thistles is an illustration to show just how impossible it is for the false teacher to have the reconciling attitude that the Sermon demands…nothing productive is suggested in the Sermon by the thorns and thistles, which are not only nonproductive, but destructive of the crop. The community of the followers of Jesus is not to expect anything good from the false prophets who sap it of its strength….Matthew mixes the metaphor. Rotten trees do not bring forth rotten fruit, but evil fruit, i.e., works which Satan, the Evil One, is causing.
In the light of the Sermon, these fruits are love of the neighbor, reconciliation with the enemy, and performing good to all people indiscriminately. Limiting these fruits to mere restraint from evil and sin would only be the righteousness of the Pharisees and scribes. The false prophet may perpetrate no evil, but he refuses to be reconciled as is required by the Sermon.
On verses 21-23:
Here the Sermon calls the false prophets workers of iniquity in spite of their having prophesied, their doing great deeds, and their casting out demons in Jesus’ name. In the great judgment scene [of Matthew 25] the damned have failed to come to the aid of those who were in distress. At this point that scene provides a commentary on the Sermon’s chief requirement of reconciliation by loving the neighbor through concrete deeds. The greater righteousness is not mere ethical life, but one which does good to all people, including one’s enemies.
There can be no thought of universalism here, as if it were immaterial whether or not a person who does the will of the Father recognizes Jesus as Lord. This is impossible for three reasons. Though not everyone who calls Jesus Lord will enter the kingdom of heaven, only those who have called him Lord will indeed enter. A delimiting factor has already been introduced. Second, the ones entering do the will of Jesus’ Father. It is for the performance of the Father’s will on earth that the church prays in the Third Petition, “Thy will be done.” Within the Sermon and indeed the entire gospel, Matthew intends that God’s will should be understood as the reconciliation of all persons to himself and to each other through the preaching of the Gospel and by its actual practice. This embraces the command to love within a christological perspective. Third, this is the first reference to God as “My Father.” With this Jesus claims a special relationship to God as his Son. Only the Son has full knowledge of the Father and is his only revealer (Matt. 11:27). He is both the one whom God has chosen to reveal himself and whom God has designated as the final Judge of all people.
To act or speak in the name of Jesus was to do it in his stead and with his authorization. Such a phrase as “in my name” points not to the ordinary acts by members of the community of Jesus, but those performed in a formal, liturgical way. In view here is an established church, and not clusters of disciples gathered in informal ad hoc groupings.
The word Matthew uses for Jesus’ addressing the false prophets is “confess” (homologein), and it is used in Matt. 10:32 for the church’s creed or confession of Jesus and for his acknowledging his disciples as his own. “Confess” carries with it a high degree of certainty in which the speaker allows no doubt, or possibility for change, in what he has said. It carries with it the idea of creedal certainty…. [T]he lawless ones are the false prophets who fail to do the Father’s will revealed in Jesus. This law is different from that of the Jews but it is the law of live to which all Christians must submit. But this is not law in the ordinary sense…. The workers of lawlessness are those who have not heeded the message of reconciliation now written down as the Sermon.