Have G-d send me an email, or perhaps a phone call.
That is all that it takes.
That still wouldn't be enough. NO phone call and no email would suffice. You would not accept them as from God. You would find some way to explain them away. Even if you saw someone rise from the dead, you would not believe.
Luke 16:27-31 And [the rich man in hell] said, ‘Then I beg you, father, to send him to my father's house—for I have five brothers—so that he may warn them, lest they also come into this place of torment.’ But Abraham said, ‘They have Moses and the Prophets; let them hear them.’ And he said, ‘No, father Abraham, but if someone goes to them from the dead, they will repent.’ He said to him, ‘If they do not hear Moses and the Prophets, neither will they be convinced if someone should rise from the dead.’”
yup
Like the story about the two hikers, good friends, far up into the Tetons.
One of them took a break to relieve his bladder, and was bit on the end of his male bit by a poisonous snake. He goes into shock and passes out.
His hiking buddy whips out his cell phone, and calls 911. The dispatcher connects him with a doctor, who tells him that for his friend to live, he will have to suck the poison out.
The damaged hiker awakes from his swoon, and tells his friend to call for help.
“Already did” says his friend. They say you are going to die.