Certificates and NP?
You must be able to verify in deterministic polynomial time that the certificate is correct.

If your certificate for SAT is a truth assignment, you can verify that it's a satisfying assignment in deterministic polynomial time, by substituting the truth values of the variables and simplifying the expression. However, if your certificate was just "It's satisfiable, honest!", then you would only be able to verify that if you could solve SAT in P, which you probably can't. The certificate has to be a proof that the string is in the language, not just a claim that it is.

Indeed, if it was enough to just claim that the string is in the language, then every language would be in NP!  For example, the certificate version of the halting problem would be just the set of all strings "$\langle M, w\rangle\text{ It halts, honest!}$" for all machines $M$ and inputs $w$ such that $M(w)$ halts, and all strings "$\langle M,w\rangle\text{ This one loops - too bad!}$" for all machines $M$ and inputs $w$ such that $M(w)$ doesn't halt.