"He saved others; Himself he cannot save. If He be king of Israel, let Him now come down from the cross, and we will believe Him." Matthew 27:42 "There was a certain rich man, which was clothed in purple and fine linen, and fared sumptuously every day: And there was a certain beggar named Lazarus, which was laid at his gate, full of sores, And desiring to be fed with the crumbs which fell from the rich man's table: moreover the dogs came and licked his sores. And it came to pass, that the beggar died, and was carried by the angels into Abraham's bosom: the rich man also died, and was buried; And in hell he lift up his eyes, being in torments, and seeth Abraham afar off, and Lazarus in his bosom. And he cried and said, Father Abraham, have mercy on me, and send Lazarus, that he may dip the tip of his finger in water, and cool my tongue; for I am tormented in this flame. But Abraham said, Son, remember that thou in thy lifetime receivedst they good things, and likewise Lazarus evil things: but now he is comforted, and thou art tormented. And beside all this, between us and you there is a great gulf fixed: so that they which would pass from hence to you cannot; neither can they pass to us, that would come from thence. Then he said, I pray thee therefore, father, that thou wouldest send him to my father's house: For I have five brethren; that he may testify unto them, lest they also come into this place of torment. Abraham saith unto him, they have Moses and the prophets; let them hear them. And he said, nay, father Abraham: but if one went unto them from the dead, they will repent. And he said unto him, If they hear Moses and the prophets, neither will they be persuaded, though one rose from the dead." Luke 16:19-31 Also read John 5:39-47
Isn't it ironic that those who don't believe are always trying to tell those who do believe, what to tell those who don't believe, to convince those that do believe, not to believe?
Had Jesus walked off the cross, whomever didn't believe that Jesus was the Son of God believed, would still not believe that Jesus is the Son of God. God designed man to believe in God through several methods, all of which non-believers deny.
1 God designed each person to have, in their soul, a desire to know their creator. It is inherent.
2 God designed all creation to speak of Him through the experience of the sciences.
3 God designed each person to have a need to connect with the source of their existence.
4 God designed every person to respond to the inner call from God.
5 God confirmed His existence through the writing of the scriptures, where he used men, to confirm in writing God's self disclosure, throughout the ages. The Bible is the only book that reveals the actual genealogies of man from Jesus backward to Adam.
6 The Bible verifies and codifies the history of man from Adam to Jesus by having the Bible span about 4,000 years from Adam to Jesus with multiple writers from differing cultures and milleniums, cohesively joining together the only credible source for historically accurate anthropology. And then from Jesus until now and beyond the Bible prophesies unquestionable events that have taken place and that will take place in the future.
Even if Jesus had come down from the cross or if someone went back from heaven to earth to notify those on earth that God is real, heaven is real and hell is real, those who don't believe wouldn't believe. The non-believer is always looking for more evidence. If the evidence that is available isn't enough, well, think about it.