This exchange between Jesus and the Pharisees from Mark's gospel account (10:1-12) struck me earlier today:
10:1 And he left there and went to the region of Judea and beyond the Jordan, and crowds gathered to him again. And again, as was his custom, he taught them.
2 And Pharisees came up and in order to test him asked, “Is it lawful for a man to divorce his wife?” 3 He answered them, “What did Moses command you?” 4 They said, “Moses allowed a man to write a certificate of divorce and to send her away.” 5 And Jesus said to them, “Because of your hardness of heart he wrote you this commandment. 6 But from the beginning of creation, ‘God made them male and female.’ 7 ‘Therefore a man shall leave his father and mother and hold fast to his wife,  8 and the two shall become one flesh.’ So they are no longer two but one flesh. 9 What therefore God has joined together, let not man separate.”
10 And in the house the disciples asked him again about this matter. 11 And he said to them, “Whoever divorces his wife and marries another commits adultery against her, 12 and if she divorces her husband and marries another, she commits adultery.”
Whole sermons can and have been preached on this, but what really struck me was how the Pharisees dodged Jesus's question, "What did Moses command you?" Notice that they answer "Moses allowed a man to write a certificate of divorce and to send her away."
The Pharisees answered Jesus by noting what they viewed as an escape clause (see Deut. 24:1-4) to God's design for marriage. Self-righteous legalists --including yours truly all too often -- have a keen eye for loopholes. But Jesus goes back to God's ordaining marriage before the Fall, to God's original command regarding family life. This command was recorded in Genesis by none other than, you guessed it, the prophet Moses.
What's more, in the two verses that follow, can see that while the Pharisees are concerned with justifying themselves by making a legal case based on technicalities, Jesus demonstrates that you can technically and outwardly follow God's law while your heart is desperately sick with sin:
10 And in the house the disciples asked him again about this matter. 11 And he said to them, “Whoever divorces his wife and marries another commits adultery against her, 12 and if she divorces her husband and marries another, she commits adultery.”Here Jesus addresses the heart motivation behind a lot of divorce common at the time. Sick of your wife? Want a younger, richer model? For the Pharisee, the answer might as well be to send the old one packing with a certificate of divorce and trade up.
Jesus makes clear that thinking doesn't cut it with God, who weighs the heart.