Then some of the Pharisees and teachers of the law said to him, "Teacher, we want to see a miraculous sign from you." Jesus answered, "A wicked and adulterous generation asks for a miraculous sign! But none will be given it except the sign of the prophet Jonah. For as Jonah was three days and three nights in the belly of a huge fish, so the Son of Man will be three days and three nights in the heart of the earth." Mt 12:38-40Whenever the gospel is preached in the New Testament, the resurrection of Jesus Christ from the dead is mentioned as validating evidence.
Acts 2:24a "God raised him from the dead"
Acts 3:15 "You killed the author of life, but God raised him from the dead. We are witnesses of this."
Acts 4:10 "It is by the name of Jesus Christ of Nazareth, whom you crucified but whom God raised from the dead, that this man stands before you healed."
Acts 10:39,40 "We are witnesses of everything he did in the country of the Jews and in Jerusalem. They killed him by hanging him on a tree, but God raised him from the dead on the third day."
Acts 13:29-31 "When they had carried out all that was written about him, they took him down from the tree and laid him in a tomb. But God raised him from the dead, and for many days he was seen by those who had traveled with him from Galilee to Jerusalem. They are now his witnesses to our people."
Acts 17:31 "God has set a day when he will judge the world with justice by the man he has appointed. He has given proof of this to all men by raising him from the dead."
Rom 1:1-4 "Paul, a servant of Christ Jesus, called to be an apostle and set apart for the gospel of God— the gospel he promised beforehand through his prophets in the Holy Scriptures regarding his Son, who as to his human nature was a descendant of David, and who through the Spirit of holiness was declared with power to be the Son of God by his resurrection from the dead: Jesus Christ our Lord."1Cor 15:1-5 "Now, brothers, I want to remind you of the gospel I preached to you, which you received and on which you have taken your stand. By this gospel you are saved, if you hold firmly to the word I preached to you. Otherwise, you have believed in vain. For what I received I passed on to you as of first importance: that Christ died for our sins according to the Scriptures, that he was buried, that he was raised on the third day according to the Scriptures, and that he appeared to Peter, and then to the Twelve. After that, he appeared to more than five hundred of the brothers at the same time, most of whom are still living, though some have fallen asleep."