"in science, it doesn't matter if something is true or not, the important thing is that it's useful."
Well if you mean true in the absolute sense, then yes, science isn't particularly concerned with whether something is absolutely true, since no finite amount of empirical results can ever demonstrate that. But then science doesn't use 'true' in the absolute sense, because in the real world nothing is true in the absolute sense, so the definition isn't useful. Hoist by your own petard, methinks.
And useful, in this context, would mean capable of logically explaining the known results and accurately anticipating future ones. Remember that, I'll come back to it later.
"for example, there is no evidence that space and time can warp like einstein said"
Yes there is, loads of it. Gravitational lensing, time dilation, all predicted by relativity (special and general), all later observed. So SR and GR explained the known results and accurately anticipated future ones, and that is why it is a useful theory of science.
"religion is the same. it doesn't matter if religions are true or not, the important thing is that they are useful."
Yes, if a religion could consistently explain existing knowledge and accurately predict new findings, it would indeed be a useful theory, whether it was true or not.
Unfortunately, "Goddidit!" explains nothing, predicts nothing, and so ultimately means nothing.
