it is not the eye witness accounts but the actual written records of the day. For example the romans actualy kept written records of who was crucified and when.
“If you seek Me, you will find Me, if you search for Me with all your heart....”
People like Dawkins point to that and say "A ha, no evidence!". But if we used the same standard on other historic figures we could equally say "where is the evidence that Julius Caesar or Alexander the Great actually existed?" The Egyptians left their records in stone and papyrus that was buried deep and dry. The Romans and Jews on parchment and wax tablets which are all long gone. Just copies of copies of copies (as with the Gospels) exist.