I have seen a remarkable YouTube video, to which I provide a link below, in which Richard Carrier (dubbed "atheist activist" by Wikipedia) summarizes his historical research on the existence of Jesus, all suggesting that there was never any such person as is described in the New Testament.
I had not, until watching the video, believed that the man Jesus never existed -- that is an entirely different issue from the question of whether any Supreme Being exists, which I don't accept. So I'm an atheist. But just like George Washington, who existed but never did a lot of the things attributed to him by his mythology (such as "chopping down the cherry tree"), I believed Jesus existed but that his reputation was seriously overblown, to the point that he would never recognize the character the Bible turned him into.
The chief reason I assumed Jesus must have been a real person was that I didn't see how a mythology about such a person could be created in such detail, in just a few decades, without the man even existing. (The oldest of the Bible Gospels, the Gospel of Mark, is believed to have been written between about 66 and 70 CE, thirty to forty years after the events it describes.) Carrier addresses that point by giving much more recent, and verifiable, examples of other mythologies equally elaborate, such as the "cargo cults" of the Pacific islands, and the Roswell UFO incident, that developed in that same amount of time.
Another argument I have heard for Jesus being real, and having really died and arisen, is that there is no previous tradition in mythology for a god dying and then coming back to life. I never bought that one: it suggests that Jesus' disciples had to "come up with that story out of nothing," which the proponents of the argument see as unlikely, but that ignores the fact that the disciples had plenty of motivation to create such a story. They didn't make up the story out of nothing: they had to start from, and work with, the fact that Jesus was dead, and they very much didn't want him to be. Carrier bats down that entire argument in a much more straightforward way: he cites numerous cases, prior to Christianity, of myths involving gods who did exactly that: die and then come back to life. (One of them, Inanna, was even crucified, and came back to life three days later.) That is, when people make the argument that a myth of a god dying and arising from the dead is unprecedented, they are simply historically wrong.
In addition to the negative evidence, which implies Jesus didn't have to have existed, Carrier also provides positive evidence of mythologization, noting that the content of letters written by the Apostle Paul to early Christian churches, which (aside from the Gospels) account for a significant part of the New Testament, letters that were all written before any of the Gospels, all portray a Jesus of a very different nature from the Jesus described in the Gospels written a generation later -- a Jesus who existed as a spirit, not a man who walked the Earth. (That is, the letters held to have actually been written by Paul do that. The ones that many Bible scholars hold to be forgeries, written much later, are sometimes more in line with the Gospels. Keep in mind: It's not just Carrier who says those are forgeries. He's not making that up.)
If the above arguments against the existence of Jesus don't seem strong enough, there is another thing to remember: Carrier doesn't claim to have "proved" Jesus didn't exist. But his findings introduce considerable room for doubt in the existence of an authentic Jesus, and they do refute the most often-used arguments in favor of his existence.
Finally, it is important to note that I haven't come close to listing all of Carrier's points here: I have only mentioned the highlights of a video that was itself a summary of the work. Carrier spent six years in wide-ranging research for this project and wrote it up in a 700-page book, with copious references and footnotes. Here is the video: "Did Jesus Even Exist? - The Case For Mythicism".