> How does anyone know if Jesus Christ ever really existed at all
The historical consensus appears to be that he did, with evidence on roughly the same level as other ancient figures who weren't royalty or authors.
I'd find it rather satisfying if the evidence suggested there never was any such person. But what swayed me wasn't so much the textual evidence, which is as sparse as you'd expect for anybody who wasn't a king. It was the rather embarrassing execution. Nobody spinning a triumphant messiah out of whole cloth would have included that plot point, and it's such a gaping hole they basically invented Christianity to paper over it. Plus itinerant messiahs were kind of a dime a dozen back then; it's less whether he ever lived than what makes him stand out from the crowd.
Of course conceding that there may well have been such a person does not oblige anyone to believe in healings, resurrections, violations of gravity, or any other hokum. And if it makes you feel any better, if Jesus really did live, as a devout 1st century Jew he would find the last 2,000 years of blasphemy in his name absolutely horrifying.
Edit: Further reading and the disclaimer that I'm less an expert than just a rando who occasionally browses /r/askhistorians.
> Citations?
This book was written by several different Biblical scholars, one is a pHD. Written specifically to point out the fallacies and flat out lies in Bart Ehrman's defense of the historical Jesus:
http://www.amazon.com/Ehrman-Quest-Historical-Jesus-Nazareth-ebook/dp/B00C9N0WBI/
If you're really interested. Read both (Bart Ehrman's book and the criticism) and take your own conclusions.