I'm Catholic, so that quickly answers that. Furthermore, I find there to be many metaphysical arguments to the existence of God. Aside from that, the necessity of hard proof is questionable.
For religion, sure, hard proof is not necessary at all. After all, faith is only required when you don't have proof. When you have proof, you don't need faith anymore. But for scientific knowledge, yeah, a hard proof is needed (or at least many pieces of evidence that point to a thing being the most plausible).
Many of our scientific principles stand on probable (but not absolute) hypotheses.
Yeah, they start as hypotheses. Then they're tested enough times and put against enough contrary ideas for them to become theories. No scientific principles stand only on speculation without testing and experimenting.