Although this is not part of the interesting religio-philosophical discussion we're all having here (which, BTW, finally fulfills my own dream of visiting 17th century Europe), that is simply not true. We are not sure it can be done, and we are even less sure it can be done with current preservation technology. There are few things in biology we are sure of, so many things that we only partly understand, and human neurobiology is very high on the list of things we understand least of. Also, your usage of the word "sure" sounds to me like saying, "well, we're sure God exists, we just don't know if He's Christian, Muslim or Jewish". After all, even if that were true, our actions must depend heavily on that second part of the sentence.
Finally, even if the god of cryonics is real, and even if he's Alcory, the preserved don't know if they're buying a ticket to heaven or to hell (see Cold Lazarus[1]).
Cryonics as practiced by people requesting to be frozen is a religion, but I'm not dismissing it, because I don't dismiss religion. Unlike Richard Dawkins, I don't see humanity thriving without religion, and I think the debate is theoretical, because I don't think humans can exist without creating religion.
> Should we just throw in the towel and stop researching it?
Of course not! We didn't stop studying the stars, and we ended up learning a lot from doing so -- but that doesn't make astrology not a religion. There's science and there's religion. Studying cryogenics is science; freezing yourself in the hope of being resurrected is religion. BTW, while we learned a lot from studying the stars while abandoning the religion surrounding them, we're still not much closer to reaching them than we were 200 years ago.
> Do you claim that everyone betting on new technologies are religious?
No. I might buy stocks in a company making a more efficient microwave oven (shit, I'm betting on my own actual startup). But when the matter at hand is our existential dread, and people change their behavior based on what is currently classified as bona-fide science fiction, become emotionally involved, and preach the great rewards to be bestowed on the faithful -- imagining, all the while a bright future while completely disregarding other possibilities -- then, yeah, you're looking at a religion.
This is what people are doing by having their bodies frozen after death; they are buying stake in some technology in hopes they will receive a return someday.