“We are not stoning you for any of these,” replied the Jews, “but for blasphemy, because you, a mere man, claim to be God.”
“blasphemy” = blasphemia = slander, detraction, speech injurious to another’s (especially God’s) good name. The English definition is “profane or mocking speech, writing, or action concerning God or anything regarded as sacred.” (Webster’s New Universal Unabridged Dictionary) The Jews thought Jesus to be blaspheming God by declaring himself to be equal with God and thus lowering God’s status to that of a mere man. What they didn’t stop to consider was that His claims might be true. As McClymont points out, “In reality He, being God, had become man.” Jesus was not a man trying to make himself God; in fact, the very reverse was true.
The penalty for blasphemy was death by stoning (Lev 24:15-16) but these Jews were taking the Law into their own hands by their attempt to judge and execute Jesus without a trial.
Leviticus 24:15-16 Say to the Israelites: ‘If anyone curses his God, he will be held responsible; anyone who blasphemes the name of the LORD must be put to death. The entire assembly must stone him. Whether an alien or native-born, when he blasphemes the Name, he must be put to death.
Blasphemy was ultimately the crime for which the Jews condemned Jesus (Mt 26:63-66).
Matthew 26:63-66 But Jesus remained silent. The high priest said to him, “I charge you under oath by the living God: Tell us if you are the Christ, the Son of God.” “Yes, it is as you say,” Jesus replied. “But I say to all of you: In the future you will see the Son of Man sitting at the right hand of the Mighty One and coming on the clouds of heaven.” Then the high priest tore his clothes and said, “He has spoken blasphemy! Why do we need any more witnesses? Look, now you have heard the blasphemy. What do you think?” “He is worthy of death,” they answered.