Now, I am really not trying to be snarky here, so please hear me out. Try to honestly ask yourself the question:
what evidence would make you reasonably sure that a UFO claim is in fact an alien spacecraft?
Would it take 1 photo or 100? Would it take 4 testimonials from eyewitnesses, or 50? Would it depend who reports it, like the New York Times or NASA? Would you need someone to stake their reputation on it, or their life? Would you require physical evidence, or would a report suffice? What would it take? If you really wrap your head around this question, and come up with a possible list of evidence that would possibly sway your opinion, as well as the reasons why that evidence would be convincing, you'll start to understand what an atheist thinks of the resurrection of Jesus.
When the day is done, the only evidence for the resurrection comes from (at best) four (so-called) eyewitness testimonials written decades after the events and the possible (although extremely dubious) reports of a handful of people believing so strongly they they laid their life down for it. There is no physical evidence, and no more than a handful of extra-biblical references (again, with dubious or weak content). From all of the arguments I've heard, this seems convincing to Christians.
Compared to UFO reports, for every eye-witness claimed for the resurrection, I can produce 100 witnesses for alien spacecraft - with direct reports within hours of the events. For every supposed apostle who is claimed to have been martyred, I can produce several people giving their lives for the belief in alien spacecraft (think, Heaven's Gate here). For every extra-biblical reference, I can give you thousands of photos, metal implants and alien abduction therapy session reports - all of which is stronger evidence than any of the extra-biblical references to Jesus (some of which are forged). Yet, despite all of this, I am not at all convinced that UFOs are alien spacecraft, and neither are most people.
Why is it then, that people are convinced by the --far less-- evidence for the resurrection? What's the difference?