Matthew 16:4 (KJV) 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.
Jesus Christ did many sings and wonders. He still is performing them now through His people, the church. There are people who seek after signs, wanting to see them but even if they see them the LORD knows their hard hearts and they wound not believe.

Jesus Christ explained here that the sign of Jonas is sufficient to know Him (“John 14:6-7 (KJV) Jesus saith unto him, I am the way, the truth, and the life: no man cometh unto the Father, but by me. [7] If ye had known me, ye should have known my Father also: and from henceforth ye know him, and have seen him.”) and be reconciled to GOD. The sign of Jonas is the death and resurrection of Jesus Christ which is testified by His disciples since the birth of the church. “Matthew 16:21 (KJV) From that time forth began Jesus to shew unto his disciples, how that he must go unto Jerusalem, and suffer many things of the elders and chief priests and scribes, and be killed, and be raised again the third day.”.

Therefore be encouraged today that all we need to believe in, to be saved, is Jesus Christ and His death and resurrection. “Mark 1:15 (KJV) And saying, The time is fulfilled, and the kingdom of God is at hand: repent ye, and believe the gospel.”
Signs and wonders point to Jesus Christ and it is in Him we need to put our trust in. Jesus Christ is the author of our faith and the cross is the most important sign.
“Hebrews 12:1-4 (KJV) Wherefore seeing we also are compassed about with so great a cloud of witnesses, let us lay aside every weight, and the sin which doth so easily beset us, and let us run with patience the race that is set before us, [2] Looking unto Jesus the author and finisher of our faith; who for the joy that was set before him endured the cross, despising the shame, and is set down at the right hand of the throne of God. [3] For consider him that endured such contradiction of sinners against himself, lest ye be wearied and faint in your minds. [4] Ye have not yet resisted unto blood, striving against sin.”