What does Matthew 12:39 mean?
Some scribes and Pharisees have asked Jesus for a sign. Presumably, they wanted to see Him do some specific miracle, different from those He has already done, in order to convince them He was the Messiah. Jesus instantly and flatly refuses them.The reason for this rejection is that Christ has already performed countless miracles right in front of these skeptics. They've chosen to reject the enormous evidence in front of them (John 5:39–40), no matter how ridiculous the excuse (Matthew 12:22–24). Based on how they have responded to those displays of God's power in Him, why would Jesus give them more (Matthew 7:6)? Instead, Jesus declares these hecklers part of an evil and adulterous generation.
Asking for "even more" signs from God, when He has provided so much already, is evidence of hard-hearted and obstinate disbelief (Romans 1:18–20; Psalm 19:1). God does not give signs on demand to prove Himself, but only out of His own grace and in His own timing. And what He gives is more than enough. The claim "if God would do this exact thing, then I would believe" is nothing more than a deceptive excuse.
Jesus doesn't merely condemn the Pharisees, but also the wider culture of His day. Overall, the people of Israel, both in that era and today, have rejected Jesus as the Messiah. He calls the nation around Him "adulterous," using the Old Testament metaphor for spiritual unfaithfulness to God. Since Jesus is the Son of God, the true Messiah, rejecting Him is similar in a way to rejecting one's own spouse for another (John 3:36; Ezekiel 16:32).
To those with such a deliberately resistant view, Jesus says no sign will be given. The only further sign these skeptical people—specifically meaning those of Jesus' era—will be shown is the sign of the prophet Jonah. Reading His words after His resurrection from the dead (John 2:18–22), the meaning becomes clear. Christ's impending resurrection is the sign this generation of Israelites should take notice of for evidence that Jesus is truly the Son of God.