What followed the Lord's rebuke was amazing! The Scribes and Pharisees wanted Jesus to show them a sign (Mt. 12:38)! He had just shown them a sign, and they had rejected it! In response to their ludicrous request, Jesus "answered and said unto them, An evil and adulterous generation seeketh after a sign; and there shall no sign be given to it, but the sign of the prophet Jonas: for as Jonas was three days and three nights in the whale's belly; so shall the Son of man be three days and three nights in the heart of the earth. The men of Nineveh shall rise in judgment with this generation, and shall condemn it: because they repented at the preaching of Jonas; and, behold, a greater than Jonas is here" (Mt. 12:39-41).
Jesus informed them that they had seen their last manifestation of the power of the Holy Spirit working through Him. The next sign they would see would be His death, burial, and resurrection. In other words, they had missed their opportunity to believe in Him. By rejecting the evidence provide for them by the power of the Holy Spirit working through Him, they had, once and for all, rejected the only way they could be saved from their sins: by faith in Him (Jn. 14:6).
John's Gospel reveals that the Holy Spirit's "coming and going" is unpredictable (Jn. 3:8). From the Lord's statement, it is clear that there will come a time when it will be too late to accept Him. I base this on the fact that the Jews require a sign in order to believe (1 Cor. 1:22). If there comes a time when they will no longer receive a sign, then those individuals can no longer be saved. If you are under conviction that you need to trust in Jesus as your Lord and Savior, don't put it off! There will come a time when the Spirit will no longer "pass your way." That is why the Word of God says in 2 Corinthians 6:2 -
"For He saith, I have heard thee in a time accepted, and in the day of salvation have I succoured thee: behold, now is the accepted time; behold, now is the day of salvation!"