Why was Jesus upset with the Pharisees who were asking for a sign?

Mt 12:

38 Some of the Pharisees and teachers of the law said to him, “Teacher, we want to see a sign from you.”

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They demanded a miracle.

39 He answered, “A wicked and adulterous generation asks for a sign! But none will be given it except the sign of the prophet Jonah.

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They had already witnessed numerous signs and miracles performed by Jesus, such as healing the sick and casting out demons (Mt 8:16). Despite this, they demanded yet another sign, revealing their hardened hearts and refusal to believe.

They had ulterior motives. Mk 8:

11 The Pharisees came and began to question Jesus. To test him, they asked him for a sign from heaven. 12 He sighed deeply and said, “Why does this generation ask for a sign? Truly I tell you, no sign will be given to it.”

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He warned them against seeking signs for the wrong reasons and a call to genuine faith and repentance. Faith does not rely on constant miraculous proofs but trusts in God's word and his promises. J 20:

29 Jesus said to [Thomas], “Because you have seen Me, you have believed; blessed are those who have not seen and yet have believed.”

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That's true faith.

Why was Jesus upset with the Pharisees who were asking for a sign?

Jesus was upset with the Pharisees because their request for a sign was not a sincere inquiry but a manipulative attempt to undermine him. It revealed their lack of faith, their hypocrisy, and their hardened hearts. Jesus' refusal to grant their request served as a testament to their spiritual condition and their rejection of the truth.