This expression is not specific for issues around marriage and divorce. It is to indicate a general opposition to God’s commandments. We more than once find the expression in the Old Testament as typical of the uncircumcised heart (Deuteronomy 10:16; Jeremiah 4:4). When the people refuse to listen to the Lord the forehead is hard and the heart stubborn (Ezekiel 3:7). This attitude of unbelief was present already at the time in the desert and meant that the people could not enter the rest of God (Psalm 95:8–11). In Psalm 95 the hardness of heart is shown precisely herein, that people put the Lord to the test and put him to the proof, although they had seen his work. This is also the situation of the testing
Pharisees with respect to Jesus. He connects the command of the certificate of divorce to the general characteristic of the people, which he recognizes also in those who pose the question (your hardness of heart). The hardness of heart, as a closing of one’s heart to the Lord, his word and his work, means under the old covenant that the people go into exile and are sent away.1 The word sounds, also in the response to the Pharisees, very threatening. They worry whether a man may send his wife away, but a people with a hardened heart is sent away by the Lord.2
5 And Jesus said to them, “Because of your hardness of heart he wrote you this commandment.