Jesus had dealt with the subject of taking oaths in the Sermon on the Mount (5:33-37). He had called His critics blind guides before too (15:14). Here is a specific example of what Jesus condemned in the second woe (v. 15). By differentiating between what was binding in their oaths and what was not, the Pharisees and teachers of the law were encouraging evasive oaths that amounted to lying. Jesus' point was that people should tell the truth. Jesus condemned His critics for mishandling the Scriptures that they claimed to defend and expound.
Verses 20-22 provide the rationale for 5:33-37. Whenever a Jew took an oath he connected it in some way with God. All their oaths were therefore binding. Jesus disallowed all evasive oaths. He viewed them as untruthful speech.
