That is where personal testimonies are essential. When Jesus left this Earth, Mankind has still kept Him on trial in the Court of Public Opinion. We are then necessarily dependent on the real life experiences of those who followed Him through persecutions and even death - working class people who had had there doubts but came to know for a certainty his teachings were true and His resurrection was true, and His promises were actively true. (I john expresses this well as well as Hebrews 12. Personal experiences through personal interaction are available to everyone.