Under this atheist's logic no one can prove anything by writing a testimony down today, because tomorrow he will reject it. He is saying because the people who wrote their testimonies of Jesus are dead, we can't really prove they actually knew and saw him. By his rule we can't prove Hitler existed--those photos and written reports by people who are now dead can't be trusted.
Prove Caesar existed. Prove Nero existed.
Funny how the Bible, well documented and verified, is given less credibility than other ancient documents and events that have far less to back them up.