PASSAGE: "Some Pharisees came, and to test him they asked, 'Is it lawful for a man to divorce his wife?' He answered them, 'What did Moses command you?' They said, 'Moses allowed a man to write a certificate of dismissal and to divorce her.' But Jesus said to them 'Because of your hardness of heart he wrote this commandment for you. But from the beginning of 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 they are no longer two, but one flesh. Therefore what God has joined together, let no one put asunder.' Then in the house the disciples asked him again about this matter. He said to them, 'Whoever divorces his wife and marries another commits adultery against her; and if she divorces her husband and marries another, she commits adultery'" (Mark 10: 2-12).
COMMENT: I've expanded the single verse you are asking about to include more of this important passage. It can be a mistake to focus on a few words without being aware of the context in which they are expressed.
This is, indeed, a challenging passage in the context of Jesus' overall message of love and nonjudgment. There seem to be several different themes being expressed. First of all, Jesus is denying the absolute authority of the law as recorded in the name of Moses—the extensive legal code set down in Exodus, Leviticus and Numbers. That code allowed men to divorce their wives, but did not allow wives to divorce their husbands. Jesus notes that this law was formulated "because of your hardness of heart." In other words, it was a human misinterpretation of spiritual law. To align with spiritual law we must go back to the loving Source. So we have a key distinction between marriage laws crafted by men and the loving and creative Law of God in which true unity is attained.
The important distinction here, I think, is "what God has joined together, let no man put asunder." If a union is truly of God—if a commitment is wholeheartedly spiritual—then nothing we may do on the mortal plane can affect it. But clearly not every marriage is joined together by God. Some are entered into out for a variety of human reasons, and it may become clear that there is no spiritual union. In such a case, to remain in the relationship for the sake of human laws may be damaging to the spiritual paths and purpose of both people involved. It is, Jesus insists, "what God has joined together" that matters. And it is the Presence of God in you that can provide clarity in a challenging situation.