This hardness of heart does not point to marital unfaithfulness and similar matters but to deafness to God’s Word and work. For already at the beginning Scripture says very clearly how God himself made human beings, man and woman, into one flesh. Since the people close their hearts to God’s work in the beginning and to God’s Word about man and woman who become one flesh, they show themselves to be hard of hearing and unwilling. In that situation they separate what belongs together. And then comes the order to give at such a separation at least a certificate of divorce and later not to marry again the one sent away if in the meantime she has belonged to another man. Now the Pharisees cling to Moses in order to continue in fact the closing of their hearts to God: they want to separate what he joined together, and therein show that they remain closed to his Word and work. That same hardness of heart makes it equally easy for them to disrespect and test Jesus, the Son of God.1
5 And Jesus said to them, “Because of your hardness of heart he wrote you this commandment.