Q:How do we know if Homer,Julia Caesar, Jesus, Confucius,...

Q:
How do we know if Homer,Julia Caesar, Jesus, Confucius, Gandhi, Don Bradman, Mao Tse Tong, Napolean, Darwin, Marx, Hitler or J Howard ever existed and did what they did ?
A:
Records by eyewitness and/or reliable researchers, regardless of how old the records are
- 2 days, 2years, 20 years, 200 years, or 2000 years.
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You shouldn't bet me started on religion.

An eyewitness is a person who sees something actually happen. The general consensus is that the earliest of the gospels (Matthew or Mark depending on who you read)was probably written about 70-80AD. Whilst some people contend that it was written by one of the apostles, there's plenty to suggest that it was written by people well after Christ's time. In all liklihood we are dealing with a book written about a person whom the authors' never met.
Secondly if you go to secular sources, the information is scarce. Josephus and a couple of other people (perhaps tacitus from memory) are the only people that rate Christ worthy of a very brief mention. You must ask yourself why do Cicero or Caesar get so much attention, yet the 'son of God' gets a couple of paragraphs. As soon you apply logic, rather than faith to religion it goes out the door. A bit like TZL in many ways.
If you're going to base your whole life on these few pieces of information that's fine, but don't be surprised when you get to the end and there's nothing there.