“No sign except the sign of Jonah”
A wicked and adulterous generation demands a sign, but none will be given it except the sign of Jonah. Then He left them and went away. (Matthew 16:4 BSB)
What it sounds like today
'Adulterous' seems like a strange accusation about marriage morals.
The cultural key
'Adulterous' is covenant language, not bedroom language: since Hosea, Israel was God's bride, and unfaithfulness to him was adultery. Demanding a sign after the miracles already given is the unfaithful spouse demanding the husband prove himself. 'The heart of the earth' means the grave, Sheol; Jonah's own prayer from the fish calls it 'the belly of Sheol' (Jonah 2:2). 'Three days and three nights' follows inclusive Jewish day-counting: any part of a day counts as the day. The demand returns, now from Pharisees and Sadducees together (rival parties united by the request), and the answer repeats shortened: no explanation this time, just the name Jonah and a departure. The second refusal is briefer than the first.
Recovered meaning
The only sign left for a generation that ignored all the others is Jonah's: a man given up for dead, three days in the deep, returned alive with a summons to repent. The resurrection itself.