I've never really looked to see the 'proof' of Lincoln b/c theree's plenty written about him and by him. There are contemporary first hand accts.
Jesus has a bunch of 2nd hand accts that are written after his death. But, ironically, I got a book on Aesop's Fables and his actual existence is undocumented. But, the moral of the stories are still recorded and written down-whether or not an actual human lived named Aesop that told the stories...that's not the important part...unless you're into antiquities.
Anthony B- Socrates, Aristotle, Plato, Siddharta Buddha, if no evidence of them is ever found, it would not disprove the existence of an entity that's supposedly active in each person's life. The teachings are still relevant. Savvy?