What does Matthew 19:8 mean?

When asked about divorce by the Pharisees, Jesus has revealed to them the heart of God. Marriage without divorce is the plan of God. It's how He designed the creation to work. He made humans as male and female to be united in marriage by Himself. Jesus has shown them that God's will is that what He has joined should not be separated (Matthew 19:1–6).

The Pharisees have pointed out, though, that divorce was a reality in Israel under Moses. It is not forbidden in the law. In fact, the law pictures a scenario in which a husband gives his wife a certificate of divorce because he finds something "indecent" in her (Matthew 19:7). The Pharisees were divided amongst themselves about what qualified as indecent enough to divorce one's wife. Was it only marital infidelity or was it anything the husband simply did not like about her?

Jesus now answers that Moses allowed divorce because of the hardness of the Israelites' hearts. This means that God created a set of rules limiting the damage which divorce might cause. Jesus adds that it was not so from the beginning. In other words, God's intention in creation was that all human marriage between man and woman should be lifelong. To separate what God had joined was a violation of God's design. Under Moses, however, Israel was allowed to break the design because of the sin-hardened hearts of the people—providing limits and restraints on the practice.

Jesus will finally answer the question the Pharisees are really asking in the following verse.