If an unbeliever is trying to deny Isaih existed, they are not likely basing their opinions on evidence and reason anyway. As father Abraham said to the rich man who wanted to warn his brothers at the end of the parable: If they will not believe the prophets, neither would they believe even if one returned from the dead.
It is interesting that you say that. When I was 14, I was thrown off my bike a second before I would have been run over by a car; I would have been severely injured or even killed. There was no obstacles and I was paying complete attention to where I was riding. At the time, I knew I was saved by something supernatural. As time past and I fell into a life of sin; never thanked God for the bike accident, I dismissed this as just lucky break.
I think unbelievers would never think twice about such an event; that is until they come to Christ. After one of the times when Jesus fed thousands, the Pharisees told Jesus that they would only believe in him if he performed a heavenly miracle; as if feeding 5,000 was not impressive enough.
Mark 8:12 He sighed deeply and said, “Why does this generation ask for a sign? Truly I tell you, no sign will be given to it.”