Leviticus 24:13-16: The LORD then said to Moses, "Take the blasphemer outside the camp, and when all who heard him have laid their hands on his head, let the whole community stone him. Tell the Israelites: Anyone who curses his God shall bear the penalty of his sin; whoever blasphemes the name of the LORD shall be put to death. The whole community shall stone him; alien and native alike must be put to death for blaspheming the LORD’S name.
Blasphemy is a pretty big net; it covers everything from making fun of God to denying God to claiming to be God – which carries the added weight of encouraging others to blaspheme. This is the crime the Pharisees accused Jesus of, starting pretty early in Luke 5:11: Then the scribes and Pharisees began to ask themselves, “Who is this who speaks blasphemies? Who but God alone can forgive sins?”
And in Matthew 26:64-66, we see that the charge eventually laid upon Jesus is that of blasphemy: Jesus said to him in reply, “You have said so. But I tell you: From now on you will see ‘the Son of Man seated at the right hand of the Power’ and ‘coming on the clouds of heaven.’” Then the high priest tore his robes and said, “He has blasphemed! What further need have we of witnesses? You have now heard the blasphemy; what is your opinion?” They said in reply, “He deserves to die!”