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]).