Then He arose from there and came to the region of Judea by the other side of the Jordan. And multitudes gathered to Him again, and as He was accustomed, He taught them again. The Pharisees came and asked Him, "Is it lawful for a man to divorce his wife?" testing Him. And He answered and said to them, "What did Moses command you?" They said, "Moses permitted a man to write a certificate of divorce, and to dismiss her." And Jesus answered and said to them, "Because of the hardness of your heart he wrote you this precept. But from the beginning of the creation, God made them male and female. For this reason a man shall leave his father and mother and be joined to his wife, and the two shall become one flesh so then they are no longer two, but one flesh. Therefore what God has joined together, let not man separate." In the house His disciples also asked Him again about the same matter. So He said to them, "Whoever divorces his wife and marries another commits adultery against her. And if a woman divorces her husband and marries another, she commits adultery."

Mark 10:1-10 NKJV

Let's take a closer look at this. Jesus was teaching and some Pharisees came to test him. They wanted to test Him, with the hope of discrediting Him. We know this because earlier in Mark it says the Pharisees were plotting to destroy Him.

And He entered the synagogue again, and a man was there who had a withered hand. So they watched Him closely, whether He would heal him on the Sabbath, so that they might accuse Him. And He said to the man who had the withered hand, "Step forward." Then He said to them, "Is it lawful on the Sabbath to do good or to do evil, to save life or to kill?" But they kept silent. And when He had looked around at them with anger, being grieved by the hardness of their hearts, He said to the man, "Stretch out your hand." And he stretched it out, and his hand was restored as whole as the other. Then the Pharisees went out and immediately plotted with the Herodians against Him, how they might destroy Him.

Mark 3:1-6 NKJV

At the time Herod Antipas ruled the area. This was the same Herod who had imprisoned John the Baptist for his views on divorce and remarriage. You see, Herod married his brother's wife. As explained in Mark 6, John the Baptist said that wasn't lawful -

For Herod himself had sent and laid hold of John, and bound him in prison for the sake of Herodias, his brother Philip's wife; for he had married her. Because John had said to Herod, "It is not lawful for you to have your brother's wife."

Mark 6:17-18 NKJV

The Pharisees may have hoped that Jesus too would be imprisoned.

Divorce was a volatile issue in first -century Judaism. There were two schools of thought on the matter. One view was that a man could divorce his wife for any reason. The other view denied divorce except on the grounds of adultery. If Jesus took either side, He may lose the support of those who support the other belief.

Jesus zeroed in on the essentials of the discussion. By stating, "What did Moses command you?", Jesus showed the issue was not rabbinical interpretations of Scripture. The issue is, what does God say. The Pharisees responded that Moses permitted man to divorce his wife. Allowing someone to do something is a lot different than commanding them to. The Pharisees were quoting Moses from Deuteronomy 24 where it says

When a man takes a wife and marries her, and it happens that she finds no favor in his eyes because he has found some uncleanness in her, and he writes her a certificate of divorce, puts it in her hand, and sends her out of his house,

Deuteronomy 24:1

Moses goes on to state that the man cannot remarry the woman if she marries someone else and her second husband also divorces her. The Pharisees were only addressing the circumstances in which a man would be permitted to divorce his wife.

Jesus explained to them that God allowed divorce because of the hardness of our hearts. A good example of that are cases where there is flagrant, unrepentant sexual immorality. There are times when a spouse will have sex outside of the marriage, and refuse to stop doing so. Under these circumstances, God has allowed a provision that allows the other spouse to leave. However, the original intent was that a man be married to his wife for life. Except for divorces that occur under legitimate Biblical grounds, remarriage after divorce is adultery. The innocent party, the spouse who did not engage in prolonged unrepentant adultery, may remarry without being guilty of adultery. Based on my understanding of Paul's writings in 1st Corinthians, I believe this is also true of a believer, whose unbelieving spouse has chosen to leave the marriage.

But to the rest speak I, not the Lord: If any brother hath a wife that believeth not, and she be pleased to dwell with him, let him not put her away. And the woman which hath an husband that believeth not, and if he be pleased to dwell with her, let her not leave him. For the unbelieving husband is sanctified by the wife, and the unbelieving wife is sanctified by the husband: else were your children unclean; but now are they holy. But if the unbelieving depart, let him depart. A brother or a sister is not under bondage in such cases: but God hath called us to peace.

1st Corinthians 7:12-15