Matthew 16:4
A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas. And he left them, and departed. A wicked and adulterous generation seeketh after a sign; and there shall no sign be given unto it, but the sign of the prophet Jonas. And he left them, and departed.
Those who seek after a sign are a wicked and adulterous generation, meaning an unfaithful, perverse and spiritually unfaithful generation. That description applies to people who turn away from God's truth toward prevailing errors in appetite and conduct. Their demand comes from unbelief and pride and does not compel a new display of power or a miracle, with the sign of Jonah as the only exception.
The sign of Jonah means that as Jonah was three days and three nights in the fish, Christ would be that time in the heart of the earth. It also means Christ's preaching as a sign to His generation, as Jonah's preaching was a warning sign to Nineveh. Nineveh repented at Jonah's warning, but Christ's generation resisted and rejected a greater messenger. Because it rejected the greater messenger, that generation will face judgment from Nineveh.
No further outward evidence was needed or could meet the need, which was spiritual renewal; Christ's words spoken with the convicting power of the Holy Spirit and the signs already given were sufficient testimony to Him. The true sign of divine origin is the gospel's inward work: grace frees people from Satan's control, brings understanding of moral truth, gives a new heart and spirit, and leads to love for God and obedience to His commandments, so that inward change and a consistent life reveal the living Saviour and the Spirit's regenerating power through the word witnesses to the Son's divine mission. The verse exposes a generation that demands signs while failing to recognize and discern the signs of the times in its own day. Those who fail to recognize those signs are in danger of perishing under the plagues poured out in God's wrath. Recognizing the signs of the times calls for faithful use of the abilities and opportunities God has entrusted.